fix arity of bug_start

This is just a trivial mistake introduced in
0f36e8fc03.

ext/-test-/bug-3571/bug.c

@@ -8,7 +8,7 @@ bug_i(RB_BLOCK_CALL_FUNC_ARGLIST(i, arg))
 }
 
 static VALUE
-bug_start(VALUE self, VALUE hash)
+bug_start(VALUE self)
 {
     VALUE ary = rb_ary_new3(1, Qnil);
     rb_block_call(ary, rb_intern("map"), 0, 0, bug_i, self);