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/acid.1

@@ -70,7 +70,7 @@ at startup; see below.
 .BI -m " machine
 Assume instructions are for the given CPU type
 (see
-.MR mach (3) )
+.MR mach 3 )
 instead of using the executable header to select
 the CPU type.
 .TP
@@ -145,7 +145,7 @@ subscripts counted from 0.
 .BI delete " list", " subscript
 .PP
 Format codes are the same as in
-.MR db (1) .
+.MR db 1 .
 Formats may be attached to (unary) expressions with
 .BR \e ,
 e.g.
@@ -230,7 +230,7 @@ Print 10 lines of source around the program address.
 .BI Bsrc( address )
 Get the source line for the program address
 into a window of a running
-.MR sam (1)
+.MR sam 1
 and select it.
 .TP
 .BI line( address )
@@ -323,7 +323,7 @@ Make the given process current.
 .TP
 .BI rc( string )
 Escape to the shell,
-.MR rc (1) ,
+.MR rc 1 ,
 to execute the command string.
 .TP
 .BI include( string )
@@ -449,7 +449,7 @@ notation)
 .BR *array .
 .PP
 Trace the system calls executed by
-.MR ls (1)
+.MR ls 1
 (neither does this one):
 .IP
 .EX
@@ -503,8 +503,8 @@ acid: cont()
 .SH SOURCE
 .B \*9/src/cmd/acid
 .SH "SEE ALSO"
-.MR mk (1) ,
-.MR db (1)
+.MR mk 1 ,
+.MR db 1
 .br
 Phil Winterbottom,
 ``Acid Manual''.