Change the "async iterable" token to "async_iterable"

This ensures the grammar says LL(1). Fixes #1490.

Author: saschanaz

index.bs

@@ -5727,7 +5727,7 @@ are known as <dfn id="dfn-object-type" export>object types</dfn>.
         StringType Null
         identifier Null
         "sequence" "&lt;" TypeWithExtendedAttributes "&gt;" Null
-        "async iterable" "&lt;" TypeWithExtendedAttributes "&gt;" Null
+        "async_iterable" "&lt;" TypeWithExtendedAttributes "&gt;" Null
         "object" Null
         "symbol" Null
         BufferRelatedType Null