Show default argument explicitly for Rexexp#match? [ci skip]

re.c

@@ -3334,7 +3334,7 @@ rb_reg_match_m(int argc, VALUE *argv, VALUE re)
 /*
  *  call-seq:
  *     rxp.match?(str)          -> true or false
- *     rxp.match?(str,pos)   -> true or false
+ *     rxp.match?(str, pos=0)   -> true or false
  *
  *  Returns <code>true</code> or <code>false</code> to indicate whether the
  *  regexp is matched or not without updating $~ and other related variables.