Remove one extraneous backtick from line 116 of 'codebase-overview.md' (#8724). (#8726)

contributing/codebase-overview.md

@@ -113,7 +113,7 @@ One way to think about it is that the condition should reflect the normal situat
 
 It is a good idea to avoid spamming the console with duplicate warnings:
 
-````js
+```js
 var warning = require('warning');
 
 var didWarnAboutMath = false;