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.

1 files changed, 9 insertions, 9 deletions

diff --git a/man/man7/mhtml.7 b/man/man7/mhtml.7
index f561fb43..77385166 100644
--- a/man/man7/mhtml.7
+++ b/man/man7/mhtml.7
@@ -19,14 +19,14 @@ mhtml \- macros for formatting HTML
 \&...
 .SH DESCRIPTION
 This package of
-.MR htmlroff (1)
+.MR htmlroff 1
 macro definitions provides convenient macros for formatting HTML.
 It is usually used along with 
-.MR troff (1)
+.MR troff 1
 macro packages such as
-.MR man (7)
+.MR man 7
 and
-.MR ms (7) .
+.MR ms 7 .
 .I Mhtml
 replaces some macros defined in the other packages,
 so it should be listed after them on the
@@ -64,7 +64,7 @@ before invoking
 Accumulate footnotes and print them at the end of the
 document under a \fBNotes\fP heading.
 These replace the macros in
-.MR ms (7) .
+.MR ms 7 .
 To emit the notes accumulated so far, invoke
 .BR .NOTES .
 .TP
@@ -75,7 +75,7 @@ and
 .B .PE
 with a PNG image corresponding to the output of
 running
-.MR troff (1)
+.MR troff 1
 on the input.
 .TP
 .B .TS\fR, \fP.TE
@@ -100,6 +100,6 @@ percent of the current output width.
 .SH FILES
 .B \*9/tmac/tmac.html
 .SH  SEE ALSO
-.MR htmlroff (1) ,
-.MR htmlroff (7) ,
-.MR ms (7)
+.MR htmlroff 1 ,
+.MR htmlroff 7 ,
+.MR ms 7