su.1: refer to mdo(1) and improve manual syntax

Running `mandoc -Tlint usr.bin/su/su.1` spotted the syntax problem.

Reviewed by: imp, ziaee
Pull Request: https://github.com/freebsd/freebsd-src/pull/1712
Closes: https://github.com/freebsd/freebsd-src/pull/1712

usr.bin/su/su.1

@@ -25,7 +25,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd March 26, 2020
+.Dd June 11, 2025
 .Dt SU 1
 .Os
 .Sh NAME
@@ -46,7 +46,7 @@ and switches to that user ID
 A shell is then executed.
 .Pp
 PAM is used to set the policy
-.Xr su 1
+.Nm
 will use.
 In particular, by default only users in the
 .Dq Li wheel
@@ -234,6 +234,7 @@ Simulate a login for root.
 .El
 .Sh SEE ALSO
 .Xr csh 1 ,
+.Xr mdo 1 ,
 .Xr sh 1 ,
 .Xr group 5 ,
 .Xr login.conf 5 ,