Change labelFormat example to show as a string value (#1680)

Small change. Wasn't immediately clear that the labelFormat is a string. For folks copy pasting the code it might help to be a little more direct.

packages/babel-plugin-emotion/README.md

@@ -236,7 +236,7 @@ be prepended automatically.
 ```javascript
 // BrownView.js
 // autoLabel: true
-// labelFormat: [filename]--[local]
+// labelFormat: '[filename]--[local]'
 const brownStyles = css({ color: 'brown' })
 ```