DOC: Correct Series.nonzero docstring

The function returns indices, not a boolean array. The description now
matches `numpy.nonzero`.
OK, I'll remove that then. By the way, here are the other places it occurs:

```
pandas/core/reshape.py:35:    >>> import pandas as pd
pandas/core/reshape.py:704:    >>> import pandas as pd
pandas/core/reshape.py:824:    >>> import pandas as pd
pandas/core/reshape.py:905:    >>> import pandas as pd
pandas/core/reshape.py:1025:    >>> import pandas as pd
pandas/core/series.py:344:        >>> import pandas as pd
pandas/core/series.py:1762:        >>> import pandas as pd
pandas/core/series.py:1795:        >>> import pandas as pd
pandas/io/stata.py:88:    >>> import pandas as pd
pandas/io/stata.py:1299:    >>> import pandas as pd
pandas/tools/plotting.py:2890:    >>> import pandas
pandas/tseries/tools.py:213:    >>> import pandas as pd
```