revert incorrect change

Author: saschanaz

index.bs

@@ -4305,7 +4305,7 @@ When they are, the effect will be as you would expect.
 
 <pre class="grammar" id="prod-AsyncIterable">
     AsyncIterable :
-        "async_iterable" "&lt;" TypeWithExtendedAttributes OptionalType "&gt;" OptionalArgumentList ";"
+        "async" "iterable" "&lt;" TypeWithExtendedAttributes OptionalType "&gt;" OptionalArgumentList ";"
 </pre>
 
 <pre class="grammar" id="prod-OptionalArgumentList">