tmac/tmac.an: define .MR in a groff compatible way

groff 1.23.0 added .MR to its -man macro package.  The NEWS file states
that the inclusion of the macro "was prompted by its introduction to
Plan 9 from User Space's troff in August 2020."  From d32deab it seems
that the name for Plan 9 from User Space's implementation was suggested
by groff maintainer G. Brandon Robinson.

Not sure if the intention was to make these definitions compatible, but
it would be nice if they were.

Currently, Plan 9 from User Space's .MR expects its second argument to
be parenthesized.  groff's .MR does not.  This results in extra
parentheses appearing in manual references when viewing Plan 9 from User
Space's manual pages on a system using groff.

man/man1/mount.1

@@ -15,7 +15,7 @@ mounts a 9P server's files into the file system.
 is typically 
 either the name of a Unix domain socket
 (see
-.MR namespace (1) )
+.MR namespace 1 )
 or the name or IP address of a machine
 serving 9P over TCP port 564.
 .PP
@@ -29,7 +29,7 @@ On Linux,
 .I mount
 uses the native 9P kernel module when present.
 Otherwise it tries to use
-.MR 9pfuse (4)
+.MR 9pfuse 4
 with the FUSE file system module.
 Using the 9P kernel module requires root access.
 FUSE can often be used by regular users.
@@ -45,7 +45,7 @@ should be invoked as
 .BR mount .
 .SH EXAMPLES
 Mount 
-.MR acme (4)
+.MR acme 4
 onto 
 .B /mnt/acme :
 .IP
@@ -64,6 +64,6 @@ cat /mnt/plumb/rules
 .br
 .B \*9/bin/unmount
 .SH SEE ALSO
-.MR intro (4) ,
+.MR intro 4 ,
 .IR intro (9p),
-.MR 9pfuse (4)
+.MR 9pfuse 4