Editorial: s/an/a before "NotAllowed"

Author: eladalon1983

index.bs

@@ -5138,7 +5138,7 @@ entails in the ECMAScript language binding.
     To throw a new {{DOMException}} with [=error name=] "{{NotAllowedError!!exception}}":
 
     <blockquote>
-        [=exception/Throw=] an "{{NotAllowedError!!exception}}" {{DOMException}}.
+        [=exception/Throw=] a "{{NotAllowedError!!exception}}" {{DOMException}}.
     </blockquote>
 
     To create a new {{DOMException}} with [=error name=] "{{SyntaxError!!exception}}":