Fix typo in readonly error message

Summary: Did you mean to declaring ... -> Did you mean to declare

Reviewed By: madgen

Differential Revision: D63753961

fbshipit-source-id: a42e4d560cc6125cb1f5c8895873e688a07cfe8f

Author: Michael Thomas

hphp/hack/src/typing/tast_check/readonly_check.ml

@@ -488,7 +488,7 @@ let call
           "annotate this typehint as a " ^ suggested_fty_str
         (* Otherwise, it's likely from a Rwitness, but we suggest declaring it as readonly *)
         else
-          "declaring this as a `readonly` function"
+          "declare this as a `readonly` function"
       in
       Typing_error_utils.add_typing_error
         ~env:(Tast_env.tast_env_as_typing_env env)

hphp/hack/test/typecheck/readonly/call.php.imp_pess_exp

@@ -9,4 +9,4 @@ This function is readonly, so it must be marked readonly at declaration time to
 ERROR: File "call.php", line 29, characters 4-12:
 This function is readonly, so it must be marked readonly at declaration time to be called. (Typing[4428])
   File "call.php", line 4, characters 10-27:
-  Did you mean to declaring this as a `readonly` function?
+  Did you mean to declare this as a `readonly` function?