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/hist.1

@@ -41,7 +41,7 @@ option enables verbose debugging printout.
 The 
 .B -d
 option causes
-.MR diff (1)
+.MR diff 1
 .B -c
 to be run for each adjacent pair of dump files, while
 .B -b
@@ -73,11 +73,11 @@ by convention, root of dump file system
 .SH SOURCE
 .B \*9/src/cmd/hist.c
 .SH SEE ALSO
-.MR yesterday (1) ,
-.MR vbackup (8)
+.MR yesterday 1 ,
+.MR vbackup 8
 .SH BUGS
 Should be called
 .IR history ,
 but
 that name is taken by
-.MR sh (1) .
+.MR sh 1 .