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/rio.1

@@ -68,15 +68,15 @@ specifies an alternative program to run when the
 .I New
 menu item is selected.
 The default is to try
-.MR 9term (1)
+.MR 9term 1
 and then to fall back to
-.MR xterm (1) .
+.MR xterm 1 .
 The
 .B \-s
 option has no effect.  It formerly set the scrolling mode for
 new windows and is recognized to avoid breaking scripts.
 See
-.MR 9term (1)
+.MR 9term 1
 for a description of scrolling behavior.
 .PP
 The
@@ -137,11 +137,11 @@ Very small windows may not be created.
 The new window is created running
 .IR termprog ,
 by default
-.MR 9term (1)
+.MR 9term 1
 or, if 
 .I 9term
 is not available,
-.MR xterm (1) .
+.MR xterm 1 .
 .TP
 .B Resize
 Change the size and location of a window.
@@ -165,7 +165,7 @@ Deleting a window causes a
 .L hangup
 note to be sent to all processes in the window's process group
 (see
-.MR notify (3) ).
+.MR notify 3 ).
 .TP
 .B Hide
 Hide a window.  Click in the window to be hidden (gunsight cursor);
@@ -241,8 +241,8 @@ starts a particular program.)
 .PP
 There is a currently a compiled-in limit of 128 hidden windows.
 .SH "SEE ALSO"
-.MR 9term (1) ,
-.MR xterm (1)
+.MR 9term 1 ,
+.MR xterm 1
 .PP
 As mentioned above,
 .I rio