docs(guide/unit-testing): use whitespace to improve readability

☆.。.：・゜☆ Merci (◜௰◝) ☆.。.：・゜☆

Closes #9572

docs/content/guide/unit-testing.ngdoc

@@ -268,8 +268,8 @@ logic, further simplifying the application logic.
 
 ```js
 myModule.filter('length', function() {
-  return function(text){
-    return (''+(text||'')).length;
+  return function(text) {
+    return ('' + (text || '')).length;
   }
 });