Fix grammatical error in a new comment

Author: DougGregor

include/swift/AST/Decl.h

@@ -5948,7 +5948,7 @@ class AbstractFunctionDecl : public GenericContext, public ValueDecl {
   /// Returns true if the function is an @asyncHandler.
   bool isAsyncHandler() const;
 
-  /// Returns true if the function if the signature matches the form of an
+  /// Returns true if the function signature matches the form of an
   /// @asyncHandler.
   bool canBeAsyncHandler() const;