testsuite: Fix up cpp23/auto-fncast11.C testcase [PR103408]

This test fails:
+FAIL: g++.dg/cpp23/auto-fncast11.C  -std=c++2b  (test for errors, line 19)
+FAIL: g++.dg/cpp23/auto-fncast11.C  -std=c++2b (test for excess errors)
because the regex in dg-error was missing an indefinite article.

2021-12-15  Jakub Jelinek  <jakub@redhat.com>

	PR c++/103408
	* g++.dg/cpp23/auto-fncast11.C: Fix expected diagnostic wording.