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

Suggested by G. Brandon Robinson.

man/man1/9.1

@@ -12,7 +12,7 @@
 .B .
 .B 9
 (from
-.IM sh (1) )
+.MR sh (1) )
 .PP
 .B 9.rc
 .I cmd
@@ -24,7 +24,7 @@
 .B .
 .B 9.rc
 (from
-.IM rc (1) )
+.MR rc (1) )
 .PP
 .B u
 .I cmd
@@ -36,7 +36,7 @@
 .B .
 .B u
 (from
-.IM sh (1) )
+.MR sh (1) )
 .PP
 .B u.rc
 .I cmd
@@ -48,7 +48,7 @@
 .B .
 .B u.rc
 (from
-.IM rc (1) )
+.MR rc (1) )
 .SH DESCRIPTION
 Because Plan 9 supplies commands with the same name as but different
 behavior than many basic Unix system commands
@@ -86,7 +86,7 @@ in order to make the current shell start running in the Plan 9 environment.
 is the same as
 .I 9
 but written for use by the shell
-.IM rc (1) .
+.MR rc (1) .
 .PP
 .I U
 and
@@ -106,7 +106,7 @@ $ 9 grep '[α-ζ]' /etc/passwd
 .EE
 .PP
 Start an
-.IM rc (1)
+.MR rc (1)
 with the Plan 9 commands in the path before the system commands,
 and then run the Unix
 .IR ls :
@@ -124,7 +124,7 @@ $ 9 rc
 .br
 .B \*9/bin/u.rc
 .SH SEE ALSO
-.IM intro (1)
+.MR intro (1)
 .SH BUGS
 Some shell configurations
 (notably, oh-my-zsh)