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

Suggested by G. Brandon Robinson.

man/man1/yacc.1

@@ -23,7 +23,7 @@ to produce a program
 This program must be loaded with a lexical analyzer function,
 .B yylex(void)
 (often generated by
-.IM lex (1) ),
+.MR lex (1) ),
 with a
 .B main(int argc, char *argv[])
 program, and with an error handling routine,
@@ -115,7 +115,7 @@ option reverses this.
 The parser accepts
 .SM UTF
 input text (see
-.IM utf (7) ),
+.MR utf (7) ),
 which has a couple of effects.
 First, the return value of
 .B yylex()
@@ -156,7 +156,7 @@ parser prototype using stdio
 .SH SOURCE
 .B \*9/src/cmd/yacc.c
 .SH "SEE ALSO"
-.IM lex (1)
+.MR lex (1)
 .br
 S. C. Johnson and R. Sethi,
 ``Yacc: A parser generator'',