patch 8.2.1626: test for strchars() fails with different error number

Problem:    Test for strchars() fails with different error number.
Solution:   Adjust the error number.
diff --git a/src/testdir/test_utf8.vim b/src/testdir/test_utf8.vim
index a1e97fd..bfb90c1 100644
--- a/src/testdir/test_utf8.vim
+++ b/src/testdir/test_utf8.vim
@@ -21,7 +21,7 @@
     call assert_equal(exp[i][2], strchars(inp[i], 1))
   endfor
   call assert_fails("let v=strchars('abc', [])", 'E745:')
-  call assert_fails("let v=strchars('abc', 2)", 'E474:')
+  call assert_fails("let v=strchars('abc', 2)", 'E1023:')
 endfunc
 
 " Test for customlist completion