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/page.1

@@ -25,7 +25,7 @@ of a
 PostScript,
 PDF,
 or
-.MR troff (1)
+.MR troff 1
 or
 Unix's
 .IR tex (1)
@@ -41,7 +41,7 @@ graphics files
 FAX
 page,
 a Plan 9
-.MR image (7)
+.MR image 7
 file, an Inferno bitmap file, or other common format).
 .I Page
 displays these
@@ -108,7 +108,7 @@ it listens to the
 .B image
 plumbing channel
 (see
-.MR plumber (4) )
+.MR plumber 4 )
 for more images to display.
 The
 .B -i
@@ -194,7 +194,7 @@ reverses the order in which pages are displayed.
 Typing a
 .B w
 will write the currently viewed page to a new file as a compressed
-.MR image (7)
+.MR image 7
 file.
 When possible, the filename is of the form
 .IR basename . pagenum . bit .
@@ -217,7 +217,7 @@ and
 PDF
 .IR files .
 It also calls a variety of conversion programs, such as those described in
-.MR jpg (1) ,
+.MR jpg 1 ,
 to convert the various raster graphics formats
 into Inferno bitmap files.
 Pages are converted ``on the fly,'' as needed.
@@ -235,11 +235,11 @@ Browse the Inferno bitmap library.
 man -t page | page -w
 Preview this manual in a new window.
 .SH "SEE ALSO
-.MR gs (1) ,
-.MR jpg (1) ,
-.MR proof (1) ,
-.MR tex (1) ,
-.MR troff (1)
+.MR gs 1 ,
+.MR jpg 1 ,
+.MR proof 1 ,
+.MR tex 1 ,
+.MR troff 1
 .SH SOURCE
 .B \*9/src/cmd/page
 .SH DIAGNOSTICS
@@ -277,7 +277,7 @@ There are too many keyboard commands and menu items.
 Displaying a PostScript or PDF file depends both on having
 GhostScript
 (see
-.MR gs (1) )
+.MR gs 1 )
 installed and on the underlying operating system
 providing a file descriptor device tree at
 .BR /dev/fd .