Fix more “a”/“an” typos

compiler/rustc_middle/src/ty/util.rs

@@ -681,7 +681,7 @@ impl<'tcx> ty::TyS<'tcx> {
     }
 
     /// Checks whether values of this type `T` implement the `Freeze`
-    /// trait -- frozen types are those that do not contain a
+    /// trait -- frozen types are those that do not contain an
     /// `UnsafeCell` anywhere. This is a language concept used to
     /// distinguish "true immutability", which is relevant to
     /// optimization as well as the rules around static values. Note