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

Suggested by G. Brandon Robinson.

man/man1/ascii.1

@@ -91,7 +91,7 @@ control characters or insert newlines.
 is similar; it converts between
 .SM UTF
 and character values from the Unicode Standard (see
-.IM utf (7) ).
+.MR utf (7) ).
 If given a range of hexadecimal numbers,
 .I unicode
 prints a table of the specified Unicode characters \(em their values and
@@ -126,7 +126,7 @@ The file
 contains a
 table of characters and descriptions, sorted in hexadecimal order,
 suitable for
-.IM look (1)
+.MR look (1)
 on the lower case
 .I hex
 values of characters.
@@ -154,7 +154,7 @@ table of characters and descriptions.
 .br
 .B \*9/src/cmd/unicode.c
 .SH "SEE ALSO"
-.IM look (1) ,
-.IM tcs (1) ,
-.IM utf (7) ,
-.IM font (7)
+.MR look (1) ,
+.MR tcs (1) ,
+.MR utf (7) ,
+.MR font (7)