Merge pull request basarat#602 from iamhabbeboy/patch-1

Fixed italic sub heading text to maintain consistency in Null vs Undefined section

Author: basarat

docs/styleguide/styleguide.md

@@ -172,11 +172,11 @@ let foo: { x: number, y?: number } = { x:123 };
 
 * Use `undefined` in general (do consider returning an object like `{valid:boolean, value?:Foo}` instead)
 
-***Bad***
+**Bad**
 ```ts
 return null;
 ```
-***Good***
+**Good**
 ```ts
 return undefined;
 ```