docs/GeneratorExtensions.md: Fix MD036 warnings

The following warnings are generated by using markdownlint analysis:
```
MD036/no-emphasis-as-heading Emphasis used instead of a heading [Context: "sections/gen-section.h"]
MD036/no-emphasis-as-heading Emphasis used instead of a heading [Context: "sections/gen-myvendor.c"]
```
Refer to markdown-lint [1] to fix MD036
[1]: https://github.com/updownpress/markdown-lint/blob/master/rules/036-no-emphasis-as-header.md

Change-Id: Ib39b6fff70283c7db070030614a05c3a47a859ae
Signed-off-by: George Liu <liuxiwei@ieisystem.com>
diff --git a/docs/GeneratorExtensions.md b/docs/GeneratorExtensions.md
index 4b189cd..128e888 100644
--- a/docs/GeneratorExtensions.md
+++ b/docs/GeneratorExtensions.md
@@ -12,7 +12,7 @@
 sake of example, we will create a generator for a fake OEM section
 "myVendorSection".
 
-_sections/gen-section.h_
+_sections/gen-section.h_:
 
 ```c
 //Section generator function predefinitions.
@@ -22,7 +22,7 @@
 size_t generate_section_myvendor(void** location);
 ```
 
-_sections/gen-myvendor.c_
+_sections/gen-myvendor.c_:
 
 ```c
 /**