:nth-child(An+B) does not work when B is a 2-digit value (10, 11, 12 etc.)

When I type something like:

```
'#table1 tbody tr:nth-child(n+13)'
```

instead of selecting rows from the 13th onwards, `pup` simply selects all the rows in the table and behaves like:

```
'#table1 tbody tr:nth-child(n)'
```

However, when B is single digit value, it behaves normally.