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/man1/9.1

@@ -12,7 +12,7 @@
 .B .
 .B 9
 (from
-.MR sh (1) )
+.MR sh 1 )
 .PP
 .B 9.rc
 .I cmd
@@ -24,7 +24,7 @@
 .B .
 .B 9.rc
 (from
-.MR rc (1) )
+.MR rc 1 )
 .PP
 .B u
 .I cmd
@@ -36,7 +36,7 @@
 .B .
 .B u
 (from
-.MR sh (1) )
+.MR sh 1 )
 .PP
 .B u.rc
 .I cmd
@@ -48,7 +48,7 @@
 .B .
 .B u.rc
 (from
-.MR rc (1) )
+.MR rc 1 )
 .SH DESCRIPTION
 Because Plan 9 supplies commands with the same name as but different
 behavior than many basic Unix system commands
@@ -86,7 +86,7 @@ in order to make the current shell start running in the Plan 9 environment.
 is the same as
 .I 9
 but written for use by the shell
-.MR rc (1) .
+.MR rc 1 .
 .PP
 .I U
 and
@@ -106,7 +106,7 @@ $ 9 grep '[α-ζ]' /etc/passwd
 .EE
 .PP
 Start an
-.MR rc (1)
+.MR rc 1
 with the Plan 9 commands in the path before the system commands,
 and then run the Unix
 .IR ls :
@@ -124,7 +124,7 @@ $ 9 rc
 .br
 .B \*9/bin/u.rc
 .SH SEE ALSO
-.MR intro (1)
+.MR intro 1
 .SH BUGS
 Some shell configurations
 (notably, oh-my-zsh)