docs: reference NAME_MAX where we talk about filenames

docs/USER_NAMES.md

@@ -87,8 +87,8 @@ hyphen. A size limit is enforced: the minimum of `sysconf(_SC_LOGIN_NAME_MAX)`
 (typically 256 on Linux; rationale: this is how POSIX suggests to detect the
 limit), `UT_NAMESIZE-1` (typically 31 on Linux; rationale: names longer than
 this cannot correctly appear in `utmp`/`wtmp` and create ambiguity with login
-accounting) and `FILENAME_MAX` (4096 on Linux; rationale: user names typically
-appear in directory names, i.e. the home directory), thus MIN(256, 31, 4096) =
+accounting) and `NAME_MAX` (255 on Linux; rationale: user names typically
+appear in directory names, i.e. the home directory), thus MIN(256, 31, 255) =
 31.
 
 Note that these rules are both more strict and more relaxed than all of the