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/deroff.1

@@ -16,13 +16,13 @@ reads each file in sequence
 and removes all
 .I nroff
 and
-.MR troff (1)
+.MR troff 1
 requests and non-text arguments, backslash constructions,
 and constructs of preprocessors such as
-.MR eqn (1) ,
-.MR pic (1) ,
+.MR eqn 1 ,
+.MR pic 1 ,
 and
-.MR tbl (1) .
+.MR tbl 1 .
 Remaining text is written on the standard output.
 .I Deroff
 follows files included by
@@ -67,7 +67,7 @@ requests.
 Remove titles, attachments, etc., as well as ordinary 
 .IR troff
 constructs, from
-.MR ms (7)
+.MR ms 7
 or
 .I mm
 documents.
@@ -84,7 +84,7 @@ does for
 and
 .I latex
 (see
-.MR tex (1) )
+.MR tex 1 )
 files what
 .B deroff -wi
 does for
@@ -96,8 +96,8 @@ files.
 .B \*9/src/cmd/delatex.lx
 .SH "SEE ALSO"
 .IR troff (1), 
-.MR tex (1) ,
-.MR spell (1)
+.MR tex 1 ,
+.MR spell 1
 .SH BUGS
 These filters are not complete interpreters of
 .I troff