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

Suggested by G. Brandon Robinson.

man/man1/tar.1

@@ -112,14 +112,14 @@ archive entries.
 .B z
 Operate on compressed tar archives.
 The type of compression is inferred from the file name extension:
-.IM gzip (1)
+.MR gzip (1)
 for
 .B .tar.gz
 and
 .BR .tgz ;
 .I bzip2
 (see
-.IM gzip (1) )
+.MR gzip (1) )
 for
 .BR .tar.bz ,
 .BR .tbz ,
@@ -153,8 +153,8 @@ can be used to copy hierarchies thus:
 .SH SEE ALSO
 .I 9ar
 in
-.IM 9c (1) ,
-.IM bundle (1)
+.MR 9c (1) ,
+.MR bundle (1)
 .SH BUGS
 There is no way to ask for any but the last
 occurrence of a file.