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: I51aa5398d1be503d8db7af666312e7d30348b56d
diff --git a/README.md b/README.md
index 5ae5bc2..447ed88 100644
--- a/README.md
+++ b/README.md
@@ -50,7 +50,7 @@
 
 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
@@ -58,7 +58,7 @@
 
 For a test / debug build, a typical configuration is:
 
-```
+```sh
 meson -Dtests=enabled <build_dir>
 ninja -C <build_dir> test
 ```