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

Suggested by G. Brandon Robinson.

man/man1/bc.1

@@ -218,7 +218,7 @@ Assignment to
 .B scale
 influences the number of digits to be retained on arithmetic
 operations in the manner of
-.IM dc (1) .
+.MR dc (1) .
 Assignments to
 .B ibase
 or
@@ -235,7 +235,7 @@ empty square brackets must follow the array name.
 .PP
 .I Bc
 is actually a preprocessor for
-.IM dc (1) ,
+.MR dc (1) ,
 which it invokes automatically, unless the
 .B -c
 (compile only)
@@ -273,7 +273,7 @@ mathematical library
 .B \*9/src/cmd/bc.y
 .SH "SEE ALSO"
 .IR dc (1), 
-.IM hoc (1)
+.MR hoc (1)
 .SH BUGS
 No
 .LR && ,