DOC: update the Period.start_time attribute docstring
Checklist for the pandas documentation sprint (ignore this if you are doing
an unrelated PR):

- [x] PR title is "DOC: update the <your-function-or-method> docstring"
- [x] The validation script passes: `scripts/validate_docstrings.py <your-function-or-method>`
- [x] The PEP8 style check passes: `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] The html version looks good: `python doc/make.py --single <your-function-or-method>`
- [ ] It has been proofread on language by another sprint participant

Please include the output of the validation script below between the "```" ticks:

```
(pandas_dev) root@kalih:~/pythonpanda/pandas# python scripts/validate_docstrings.py pandas.Period.start_time

################################################################################
##################### Docstring (pandas.Period.start_time) #####################
################################################################################

Get the Timestamp for the start of the period.

Returns
-------
Timestamp

See also
--------
Period.dayofyear : Return the day of year.
Period.daysinmonth : Return the days in that month.
Period.dayofweek : Return the day of the week.

Examples
--------
>>> period = pd.Period('2012-1-1', freq='D')
>>> period
Period('2012-01-01', 'D')

>>> period.start_time
Timestamp('2012-01-01 00:00:00')
>>> period.end_time
Timestamp('2012-01-01 23:59:59.999999999')

################################################################################
################################## Validation ##################################
################################################################################

Errors found:
	No extended summary found


```

If the validation script still gives errors, but you think there is a good reason
to deviate in this case (and there are certainly such cases), please state this
explicitly.
