DOC: update the Period.dayofyear docstring
Signed-off-by: Tushar Mittal <chiragmittal.mittal@gmail.com>

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>`
- [x] It has been proofread on language by another sprint participant

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

```
################################################################################
##################### Docstring (pandas.Period.dayofyear)  #####################
################################################################################

Return the day of the year.

This attribute returns the day of the year on which the particular
date occurs. The return value ranges between 1 to 365 for regular
years and 1 to 366 for leap years.

Returns
-------
int
    The day of year.

See Also
--------
Period.dayofweek : Return the day of week.
Period.daysinmonth : Return the days in that month.
PeriodIndex.dayofyear : Return the day of year of all indexes. 

Examples
--------
>>> period = pd.Period("2015-10-23", freq='H')
>>> period.dayofyear
296
>>> period = pd.Period("2012-12-31", freq='D')
>>> period.dayofyear
366
>>> period = pd.Period("2013-01-01", freq='D')
>>> period.dayofyear
1

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

Docstring for "pandas.Period.dayofyear" correct. :)
```

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.

@jreback please review :slightly_smiling_face:  made the required changes, please review @jorisvandenbossche :slightly_smiling_face:  > Apart from the discussion about start/end in the summary line

Let's merge this, and will open a new issue to further discuss the confusion around start/end of the period. We can later fix the wording of the summary line. @techytushar Thanks a lot! I opened an issue here: https://github.com/pandas-dev/pandas/issues/20324