doc: Mention gnulib putenv module?

[Collin Funk]

Is there a reason why the Gnulib putenv module isn't mentioned on this
line or can I apply this diff with a ChangeLog entry?

diff --git a/doc/posix-functions/putenv.texi b/doc/posix-functions/putenv.texi
index b0ca4ab8b5..d9e15f5e38 100644
--- a/doc/posix-functions/putenv.texi
+++ b/doc/posix-functions/putenv.texi
@@ -4,7 +4,7 @@ @node putenv
 
 POSIX specification:@* 
@url{https://pubs.opengroup.org/onlinepubs/9699919799/functions/putenv.html}
 
-Gnulib module: ---
+Gnulib module: putenv
 
 Portability problems fixed by Gnulib:
 @itemize

Collin