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

Suggested by G. Brandon Robinson.

man/man1/ls.1

@@ -29,7 +29,7 @@ is the same as
 but sets the
 .B -p
 option and pipes the output through
-.IM mc (1) .
+.MR mc (1) .
 .PP
 There are a number of options:
 .TP
@@ -42,7 +42,7 @@ List in long format, giving mode (see below), file system type
 (e.g., for devices, the
 .B #
 code letter that names it; see
-.IM intro (3) ),
+.MR intro (3) ),
 the instance or subdevice number, owner, group,
 size in bytes, and time of last modification
 for each file.
@@ -60,7 +60,7 @@ Print only the final path element of each file name.
 List the
 .I qid
 (see
-.IM stat (3) )
+.MR stat (3) )
 of each file; the printed fields are in the order
 path, version, and type.
 .TP
@@ -99,7 +99,7 @@ otherwise.
 .TP
 .B -Q
 By default, printed file names are quoted if they contain characters special to
-.IM rc (1) .
+.MR rc (1) .
 The
 .B -Q
 flag disables this behavior.
@@ -168,5 +168,5 @@ if none of the above permissions is granted.
 .br
 .B \*9/bin/lc
 .SH SEE ALSO
-.IM stat (3) ,
-.IM mc (1)
+.MR stat (3) ,
+.MR mc (1)