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: I3fb1035541a7b47a571aba7ef4222a8868fa1940
diff --git a/README.md b/README.md
index 19bc1a7..f49620f 100644
--- a/README.md
+++ b/README.md
@@ -11,7 +11,7 @@
 
 For a standard OpenBMC release build, you want something like:
 
-```
+```sh
 meson -Dtests=disabled <build_dir>
 ninja -C <build_dir>
 ninja -C <build_dir> install
@@ -19,7 +19,7 @@
 
 For a test / debug build, a typical configuration is:
 
-```
+```sh
 meson -Dtests=enabled <build_dir>
 ninja -C <build_dir> test
 ```