Make it clear that the ":C" variable modifier expects an extended

regular expression instead of an obsolete regular expression.
Also, cross-reference re_format(7) instead of regex(3).

MFC after:	3 days

usr.bin/make/make.1

@@ -606,8 +606,8 @@ The
 modifier is just like the
 .Cm S
 modifier except that the old and new strings, instead of being
-simple strings, are a regular expression (see
-.Xr regex 3 )
+simple strings, are an extended regular expression (see
+.Xr re_format 7 )
 and an
 .Xr ed 1 Ns \-style
 replacement string.