Fixes regex for JS examples (#1591)

This PR fixes a regex example which Prism does not highlight correctly
because it is preceded by a comment.

examples/prism-javascript.html

@@ -49,7 +49,7 @@ <h2>A division operator on the same line as a regex</h2>
 
 <h2>ES6 features</h2>
 <pre><code>// Regex "y" and "u" flags
-/[a-zA-Z]+/gimyu
+var a = /[a-zA-Z]+/gimyu;
 
 // for..of loops
 for(let x of y) { }
@@ -74,4 +74,4 @@ <h2>Known failures</h2>
 
 <h3>String interpolation containing a closing brace</h3>
 <pre><code>`${ {foo:'bar'}.foo }`
-`${ '}' }`</code></pre>
+`${ '}' }`</code></pre>