Fix MD001 warnings

The following warnings are generated by using markdownlint analysis:
```
docs/oem-extension-numbering.md:6 MD001/heading-increment Heading levels should only increment by one level at a time [Expected: h2; Actual: h3]
```
Refer to markdown-lint [1] to fix MD001
[1]: https://github.com/updownpress/markdown-lint/blob/master/rules/001-header-increment.md

Signed-off-by: George Liu <liuxiwei@ieisystem.com>
Change-Id: I7d08a0b7b3f22cef58f22d56b97293688c09da53
diff --git a/docs/oem-extension-numbering.md b/docs/oem-extension-numbering.md
index 7575e9e..6309645 100644
--- a/docs/oem-extension-numbering.md
+++ b/docs/oem-extension-numbering.md
@@ -3,21 +3,21 @@
 This document describes OEM Requests to be supported using the OpenBMC OEM
 Number.
 
-### What's in the box?
+## What's in the box?
 
 - Briefly recap OEM Extension layout as described in IPMI Specification.
 - Enumerate Command codes allocated for use with the OpenBMC OEM Number.
 - For each such code, describe the associated messages, including
   representation, function, meaning, limits, and so on.
 
-### OEM Extensions, Block Transfer Transport Example
+## OEM Extensions, Block Transfer Transport Example
 
 This table and the next briefly recap OEM Extension messages as described in the
 [IPMI Specification](http://www.intel.com/content/www/us/en/servers/ipmi/ipmi-second-gen-interface-spec-v2-rev1-1.html).
 To keep it both simple and concrete, only BT Tansport is described. Please
 consult that specification for the full story.
 
-#### OEM Request
+### OEM Request
 
 Per section 11.1 of IPMI Spec
 
@@ -39,7 +39,7 @@
 - Serialize numbers larger than 1 byte LSB first - e.g., represent OEM
   Enterprise Number 49871 as `{0xcf, 0xc2, 0x00}`
 
-#### OEM Response
+### OEM Response
 
 Per section 11.2 of IPMI Spec
 
@@ -54,7 +54,7 @@
 |  5 ~ 7  |      | OEN      | 49871 | OEM Enterprise Number                           |
 | 8 ~ n+7 |      | Data     |   -   | n data bytes, encoding depends on Cmd           |
 
-### OpenBMC OEM Cmd Codes
+## OpenBMC OEM Cmd Codes
 
 ```c
 /*
@@ -74,12 +74,12 @@
 |    4    | fanManualCmd | Manual Fan Controls |
 | 5 ~ 255 | -            | Unallocated         |
 
-### I2C Device Access (Command 2)
+## I2C Device Access (Command 2)
 
 The next subsections describe command and response messages supporting the I2C
 Device Access extension OpenBMC OEM extension (command code 2).
 
-#### I2C Request Message - Overall
+### I2C Request Message - Overall
 
 |  Bytes  | Bits | Identifier    | Description               |
 | :-----: | :--- | :------------ | :------------------------ |
@@ -111,7 +111,7 @@
 - Depending on options, i2cdetect uses either quick write or 1 byte read;
   default is 1-byte read for eeprom/spd memory ranges, else quick write.
 
-#### I2C Request Message - Step Properties
+### I2C Request Message - Step Properties
 
 | Bytes | Bits | Identifier     | Description                                   |
 | :---: | :--: | :------------- | :-------------------------------------------- |
@@ -125,7 +125,7 @@
 |   2   |      | p              | Count parameter; see table                    |
 | 3:m+2 |      | wrPayload      | Nonempty iff p supplies nonzero m; see table. |
 
-##### Allowed step property combinations
+#### Allowed step property combinations
 
 | is_read | RecvLen |  p  | Size | Description                             |
 | :-----: | :-----: | :-: | :--- | :-------------------------------------- |
@@ -152,7 +152,7 @@
   - up to 32 bytes of logical payload; and
   - PEC byte.
 
-#### I2C Response Message
+### I2C Response Message
 
 | Bytes | Identifier |  Size  | Description        |
 | :---: | :--------- | :----: | :----------------- |