Fix attribute setter

Author: lukewarlow

index.bs

@@ -12055,7 +12055,7 @@ in which case they are exposed on every object that [=implements=] the interface
 
                 <dt>Otherwise</dt>
                 <dd>
-                    1.  If the conversion is to an IDL type [=extended attribute associated with|associated with=] the
+                    1.  If |attribute|'s type is [=extended attribute associated with|associated with=] the
                         [{{StringContext}}] extended attribute, then set |V| to the result of [=validate the string in context=], passing
                         [=this=], |V|, the {{StringContext}} extended attribute [=identifier=], and the [=identifier=]
                         of the [{{StringContext}}] extended attribute [=related construct=].