man: Fix typo in --print-unset option reference

We need to refer to --print-set instead of --print-unset.

Reported-by: Ferenc Wágner <wferi@debian.org>
Closes: #966110

man/dpkg-architecture.pod

@@ -85,7 +85,7 @@ using the POSIX shell B<eval>.
 
 =item B<-u>, B<--print-unset>
 
-Print a similar command to B<--print-unset> but to unset all variables.
+Print a similar command to B<--print-set> but to unset all variables.
 
 =item B<-c>, B<--command> I<command-string>