Corrects the use of an before Number to a for proper grammar. (#1896)

Author: GeorgiosVeropoulos

index.html

@@ -8994,7 +8994,7 @@ <h3>Processing actions</h3>
   of <a>getting the property</a> <code>y</code>
   from <var>action item</var>.
 
- <li><p>If <var>y</var> is not an <a>Number</a>, return <a>error</a>
+ <li><p>If <var>y</var> is not a <a>Number</a>, return <a>error</a>
   with <a>error code</a> <a>invalid argument</a>.
 
  <li><p>Set the <code>y</code> property of <var>action</var>