Editorial: fix incorrect wording in 2.9. Enumerations

Fixes #972.

Author: chansuke

index.bs

@@ -5381,7 +5381,7 @@ must not include duplicates.
     similar APIs are consistent.
 </p>
 
-The behavior when a string value that is not one a valid enumeration value
+The behavior when a string value that is not a valid enumeration value
 is used when assigning to an [=attribute=],
 or passed as an [=operation=] argument,
 whose type is the enumeration, is language binding specific.