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

Suggested by G. Brandon Robinson.

man/man1/plot.1

@@ -14,7 +14,7 @@ from the
 .I files
 or standard input,
 drawing the results in a newly created
-.IM rio (1)
+.MR rio (1)
 window.
 Plot persists until a newline is typed in the window.
 Various options may be interspersed with the
@@ -31,7 +31,7 @@ Erase the screen.
 .TP
 .BI -c " col"
 Set the foreground color (see
-.IM plot (7)
+.MR plot (7)
 for color names).
 .TP
 .BI -f " fill"
@@ -57,5 +57,5 @@ middle of the screen.
 .SH SOURCE
 .B \*9/src/cmd/plot
 .SH "SEE ALSO"
-.IM rio (1) ,
-.IM plot (7)
+.MR rio (1) ,
+.MR plot (7)