string(JSON): fix error message on invalid mode

The `GET_ARRAY` and `MEMBERS` modes were removed during development and
review of commit 8eab76eb84 (string(JSON): Adds JSON parsing support to
the string command, 2020-08-24, v3.19.0-rc1~159^2), but were
accidentally left in an error message.

Source/cmStringCommand.cxx

@@ -1106,8 +1106,8 @@ bool HandleJSONCommand(std::vector<std::string> const& arguments,
         mode != "EQUAL"_s) {
       throw json_error(
         { "got an invalid mode '"_s, mode,
-          "', expected one of GET, GET_ARRAY, TYPE, MEMBER, MEMBERS,"
-          " LENGTH, REMOVE, SET, EQUAL"_s });
+          "', expected one of GET, TYPE, MEMBER, LENGTH, REMOVE, SET, "
+          " EQUAL"_s });
     }
 
     const auto& jsonstr = args.PopFront("missing json string argument"_s);