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

Suggested by G. Brandon Robinson.

man/man3/malloc.3

@@ -132,8 +132,8 @@ the source of allocation.
 .SH SEE ALSO
 .I trump
 (in
-.IM acid (1) ),
-.IM getcallerpc (3)
+.MR acid (1) ),
+.MR getcallerpc (3)
 .SH DIAGNOSTICS
 .I Malloc, realloc
 and
@@ -153,7 +153,7 @@ The
 library for
 .I acid
 can be used to obtain traces of malloc execution; see
-.IM acid (1) .
+.MR acid (1) .
 .SH BUGS
 The different specification of
 .I calloc
@@ -182,4 +182,4 @@ are preprocessor macros defined as
 and
 .IR p9free ;
 see
-.IM intro (3) .
+.MR intro (3) .