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/db.1

@@ -45,11 +45,11 @@ specifies the memory image of a process.
 A 
 .I pid
 gives the id of an executing process to be accessed via
-.MR ptrace (2) .
+.MR ptrace 2 .
 A
 .I corefile
 specifies the name of a core dump (see
-.MR core (5)
+.MR core 5
 on your system of choice) containing the
 memory image of a terminated process.
 This manual refers to the memory image specified by
@@ -628,7 +628,7 @@ Dot is assigned to the variable or register named.
 .TP
 .B !
 The rest of the line is passed to
-.MR rc (1)
+.MR rc 1
 for execution.
 .TP
 .BI $ modifier
@@ -969,8 +969,8 @@ is one the breakpoint will fire.
 Beware that local variables may be stored in registers; see the
 BUGS section.
 .SH "SEE ALSO"
-.MR acid (1) ,
-.MR core (1)
+.MR acid 1 ,
+.MR core 1
 .SH SOURCE
 .B \*9/src/cmd/db
 .SH DIAGNOSTICS