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/ls.1

@@ -29,7 +29,7 @@ is the same as
 but sets the
 .B -p
 option and pipes the output through
-.MR mc (1) .
+.MR mc 1 .
 .PP
 There are a number of options:
 .TP
@@ -42,7 +42,7 @@ List in long format, giving mode (see below), file system type
 (e.g., for devices, the
 .B #
 code letter that names it; see
-.MR intro (3) ),
+.MR intro 3 ),
 the instance or subdevice number, owner, group,
 size in bytes, and time of last modification
 for each file.
@@ -60,7 +60,7 @@ Print only the final path element of each file name.
 List the
 .I qid
 (see
-.MR stat (3) )
+.MR stat 3 )
 of each file; the printed fields are in the order
 path, version, and type.
 .TP
@@ -99,7 +99,7 @@ otherwise.
 .TP
 .B -Q
 By default, printed file names are quoted if they contain characters special to
-.MR rc (1) .
+.MR rc 1 .
 The
 .B -Q
 flag disables this behavior.
@@ -168,5 +168,5 @@ if none of the above permissions is granted.
 .br
 .B \*9/bin/lc
 .SH SEE ALSO
-.MR stat (3) ,
-.MR mc (1)
+.MR stat 3 ,
+.MR mc 1