t/lib/warnings/utf8: Use modern name in comment

Don't confuse the reader with a historical name for a function

Author: khwilliamson

t/lib/warnings/utf8

@@ -1,7 +1,7 @@
 
   utf8.c AOK
 
-     [utf8_to_uvchr_buf]
+     [utf8_to_uv]
      Malformed UTF-8 character
 	my $a = ord "\x80" ;
 
@@ -14,7 +14,7 @@
      <<<<<< Add a test when something actually calls utf16_to_utf8
 
 __END__
-# utf8.c [utf8_to_uvchr_buf] -W
+# utf8.c [utf8_to_uv] -W
 # NAME Malformed under 'use utf8' in double-quoted string
 BEGIN {
     if (ord('A') == 193) {