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/graph.1

@@ -89,7 +89,7 @@ The next argument is
 Next argument is one or more of the characters
 .B bcgkmrwy,
 choosing pen colors by their initial letter, as in
-.MR plot (7) .
+.MR plot 7 .
 Successive curves will cycle through the colors in the given order.
 .TP
 .B  -s
@@ -145,7 +145,7 @@ is reversed.
 .B \*9/src/cmd/graph
 .SH "SEE ALSO"
 .IR plot (1), 
-.MR grap (1)
+.MR grap 1
 .SH BUGS
 Segments that run out of bounds are dropped, not windowed.
 Logarithmic axes may not be reversed.