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

Suggested by G. Brandon Robinson.

man/man3/getenv.3

@@ -18,7 +18,7 @@ int   putenv(char *name, char *val)
 fetches the environment value associated with
 .I name
 into memory allocated with
-.IM malloc (3) ,
+.MR malloc (3) ,
 0-terminates it,
 and returns a pointer to that area.
 If no file exists, 0
@@ -44,4 +44,4 @@ are preprocessor macros defined as
 and
 .IR p9putenv ;
 see
-.IM intro (3) .
+.MR intro (3) .