README.md: Fix MD040 warnings

The following warnings are generated by using markdownlint analysis:
```
MD040/fenced-code-language Fenced code blocks should have a language specified [Context: "```"]
```
Refer to markdown-lint [1] to fix MD040
[1]: https://github.com/updownpress/markdown-lint/blob/master/rules/040-fenced-code-language.md

Signed-off-by: George Liu <liuxiwei@ieisystem.com>
Change-Id: Ia972149545434ca0bf601f4b45b4792e8cdb8304
diff --git a/README.md b/README.md
index cc858c6..0251657 100644
--- a/README.md
+++ b/README.md
@@ -8,7 +8,7 @@
 
 ## Building
 
-```
+```text
 meson build/ && ninja -C build
 ```
 
@@ -16,7 +16,7 @@
 
 - Run it in OpenBMC CI, refer to [local-ci-build.md][1]
 - Run it in [OE SDK][2], run below commands in a x86-64 SDK env:
-  ```
+  ```text
   meson -Doe-sdk=enabled -Dtests=enabled build/
   ninja -C build/ test  # Meson skips running the case due to it thinks it's cross compiling
   # Manually run the tests
@@ -47,7 +47,7 @@
 
 For example:
 
-```
+```text
 meson -Dtests=disabled \
     '-DPSU_VERSION_UTIL=/usr/bin/psutils --raw --get-version' \
     '-DPSU_VERSION_COMPARE_UTIL=/usr/bin/psutils --raw --compare' \
@@ -73,7 +73,7 @@
 E.g.
 
 - Example of system with two PSUs that have different versions:
-  ```
+  ```text
    "/xyz/openbmc_project/software/02572429": {
      "Activation": "xyz.openbmc_project.Software.Activation.Activations.Active",
      "Associations": [
@@ -106,7 +106,7 @@
    },
   ```
 - Example of system with two PSUs that have the same version:
-  ```
+  ```text
    "/xyz/openbmc_project/software/9463c2ad": {
      "Activation": "xyz.openbmc_project.Software.Activation.Activations.Active",
      "Associations": [
@@ -132,7 +132,7 @@
 ### PSU update
 
 1. Generate a tarball of PSU firmware image by [generate-psu-tar tool][4].
-   ```
+   ```text
    ./generate-psu-tar --image <psu-image> --version <version> --model <model> --manufacturer \
    <manufacturer> --machineName <machineName> --outfile <psu.tar> --sign
    ```