Fix pasto, present since all recorded history.

I don't think it made any practical difference: 2, like 1, is >= 1 and
that is what all the tests of xxx_PREC test.

parse.c

@@ -485,7 +485,7 @@ EX_TREE        *parse_binary(
             if (depth >= OR_PREC)
                 return leftp;
 
-            rightp = parse_binary(cp + 1, term, 2);
+            rightp = parse_binary(cp + 1, term, OR_PREC);
             tp = new_ex_tree();
             tp->type = EX_OR;
             tp->data.child.left = leftp;