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

Suggested by G. Brandon Robinson.

man/man1/spell.1

@@ -31,7 +31,7 @@ not sanctioned there\(emon the standard output.
 .PP
 .I Spell
 ignores constructs of
-.IM troff (1)
+.MR troff (1)
 and its standard preprocessors.
 It understands these options:
 .TP
@@ -82,10 +82,10 @@ the script
 source for
 .I sprog
 .SH SEE ALSO
-.IM deroff (1)
+.MR deroff (1)
 .SH BUGS
 The heuristics of
-.IM deroff (1)
+.MR deroff (1)
 used to excise formatting information are imperfect.
 .PP
 The spelling list's coverage is uneven;