generator/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

Change-Id: I172710424721f2694c8eb5efb186db1f107279ec
Signed-off-by: George Liu <liuxiwei@ieisystem.com>
diff --git a/generator/README.md b/generator/README.md
index ece6a2e..bd30bb5 100644
--- a/generator/README.md
+++ b/generator/README.md
@@ -8,7 +8,7 @@
 
 An example usage of `cper-generator` is shown below.
 
-```
+```sh
 cper-generator --out mycper.dump --sections generic dmarvtd ia32x64 arm
 ```