tmac: rename IM (italic manual) to MR (manual reference)

Suggested by G. Brandon Robinson.

man/man1/tweak.1

@@ -35,14 +35,14 @@ If the file is a subfont, a second line presents a hexadecimal 16-bit
 .B offset
 to be applied to character values from the subfont
 (typically as stored in a font file; see
-.IM font (7) );
+.MR font (7) );
 and the subfont's
 .BR n ,
 .BR height ,
 and
 .B ascent
 as defined in
-.IM cachechars (3) .
+.MR cachechars (3) .
 .PP
 By means described below, magnified views of portions of the images
 may be displayed.
@@ -61,7 +61,7 @@ default font; the character's
 and
 .BR width
 as defined in
-.IM cachechars (3) ;
+.MR cachechars (3) ;
 and
 .BR iwidth ,
 the physical width of the image in the subfont's image.
@@ -158,9 +158,9 @@ The program will complain once about modified but unwritten files.
 .SH SOURCE
 .B \*9/src/cmd/draw/tweak.c
 .SH "SEE ALSO"
-.IM cachechars (3) ,
-.IM image (7) ,
-.IM font (7)
+.MR cachechars (3) ,
+.MR image (7) ,
+.MR font (7)
 .SH BUGS
 For a program written to adjust width tables in fonts,
 .I tweak