[print.syn] Update locking to buffered (#7168)

The function names were changed by P3235R3.

Author: frederick-vs-ja

source/iostreams.tex

@@ -4288,11 +4288,11 @@
 
   void vprint_unicode(string_view fmt, format_args args);
   void vprint_unicode(FILE* stream, string_view fmt, format_args args);
-  void vprint_unicode_locking(FILE* stream, string_view fmt, format_args args);
+  void vprint_unicode_buffered(FILE* stream, string_view fmt, format_args args);
 
   void vprint_nonunicode(string_view fmt, format_args args);
   void vprint_nonunicode(FILE* stream, string_view fmt, format_args args);
-  void vprint_nonunicode_locking(FILE* stream, string_view fmt, format_args args);
+  void vprint_nonunicode_buffered(FILE* stream, string_view fmt, format_args args);
 }
 \end{codeblock}