DOC: correct possible list lengths for `extobj` in ufunc calls

According to the docs, the list can be of lengths 1, 2, or 3, but only
length 3 appears to work:

```
>>> import numpy as np
>>> np.sin(1, extobj=[16, 1, lambda: None])
0.8414709848078965
>>> np.sin(1, extobj=[16, 1])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: UFUNC_PYVALS must be a length 3 list.
>>> np.sin(1, extobj=[16])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: UFUNC_PYVALS must be a length 3 list.
```

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