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

Suggested by G. Brandon Robinson.

man/man1/jpg.1

@@ -122,7 +122,7 @@ Typing a
 .BR q ,
 DEL, or control-D exits the program.
 For a more user-friendly interface, use
-.IM page (1) ,
+.MR page (1) ,
 which invokes these programs to convert the images to standard format,
 displays them, and offers scrolling, panning, and menu-driven navigation among the files.
 .PP
@@ -147,7 +147,7 @@ any of the following options:
 .TP
 .B -c
 Convert the image to a Plan 9 representation, as defined by
-.IM image (7) ,
+.MR image (7) ,
 and write it to standard output.
 .TP
 .B -9
@@ -156,7 +156,7 @@ Like
 but produce an uncompressed image.
 This saves processing time, particularly when the output is
 being piped to another program such as
-.IM page (1) ,
+.MR page (1) ,
 since it avoids compression and decompression.
 .TP
 .B -t
@@ -235,8 +235,8 @@ space in the image.  The icon file is written to standard output.
 .SH SOURCE
 .B \*9/src/cmd/jpg
 .SH "SEE ALSO"
-.IM page (1) ,
-.IM image (7) .
+.MR page (1) ,
+.MR image (7) .
 .SH BUGS
 Writing an animated GIF using
 .I togif