libarchive: Fix get(pw|gr)name_r comment

The commit "libarchive: Define _XOPEN_SOURCE for get(pwu|grg)id_r"
introduced a comment referring to get(pwu|grg)id_r in a source file that
actually uses the functions get(pw|gr)name_r.  We fix the comment.

Utilities/cmlibarchive/libarchive/archive_write_disk_set_standard_lookup.c

@@ -24,7 +24,7 @@
  */
 
 #ifndef _XOPEN_SOURCE
-# define _XOPEN_SOURCE 500 /* getpwuid_r and getgrgid_r signatures */
+# define _XOPEN_SOURCE 500 /* getpwnam_r and getgrnam_r signatures */
 #endif
 
 #include "archive_platform.h"