Fixed missing code block container

Summary: A missing code block closing back tick cause the documentation's layout to be incorrect.

Reviewed By: jknoxville

Differential Revision: D12948908

fbshipit-source-id: 8e9496d2bac81a49b39aed3a2c97ff6a20c71241

docs/styling-components.md

@@ -20,6 +20,7 @@ const MyView = styled('div')({
 const MyText = styled('span')({ ... });
 const MyImage = styled('img')({ ... });
 const MyInput = styled('input')({ ... });
+```
 
 ## Extending Flipper Components