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: I2b8f84817f1f46f09ff7af641584e881360d7457
diff --git a/README.md b/README.md
index 68b2270..e9f1ab0 100644
--- a/README.md
+++ b/README.md
@@ -41,8 +41,7 @@
 
 The "unrelated" group will have the default group priority of 0.
 
-```
-$ cat example.json
+```json
 {
   "leds": [
     {