DOC: Update the DatetimeIndex.normalize 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.DatetimeIndex.normalize) ##################
################################################################################

Return DatetimeIndex with times converted to midnight.

When using DatetimeIndex, the time can be converted to midnight i.e.
00:00:00. This is useful in cases, when the time does not matter.
Length is unaltered. The timezones are unaffected. If the frequency
is 'H','min' or 'S' it changes to None otherwise it remains 
unaffected.

Returns
-------
normalized : DatetimeIndex

See Also
--------
DatetimeIndex.floor : Floor the DatetimeIndex to the specified freq.
DatetimeIndex.ceil : Ceil the DatetimeIndex to the specified freq.
DatetimeIndex.round : Round the DatetimeIndex to the specified freq.

Examples
--------
>>> df = pd.DatetimeIndex(start='2014-08-01 10:00', freq='H',
...                       periods=3, tz='Asia/Calcutta')
>>> df
DatetimeIndex(['2014-08-01 10:00:00+05:30',
               '2014-08-01 11:00:00+05:30',
               '2014-08-01 12:00:00+05:30'],
                dtype='datetime64[ns, Asia/Calcutta]', freq='H')
>>> df.normalize()
DatetimeIndex(['2014-08-01 00:00:00+05:30',
               '2014-08-01 00:00:00+05:30',
               '2014-08-01 00:00:00+05:30'],
               dtype='datetime64[ns, Asia/Calcutta]', freq=None)

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

Docstring for "pandas.DatetimeIndex.normalize" 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.


Checklist for other PRs (remove this part if you are doing a PR for the pandas documentation sprint):

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20094?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@72eafde`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20094/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/20094?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #20094   +/-   ##
=========================================
  Coverage          ?    91.7%           
=========================================
  Files             ?      150           
  Lines             ?    49152           
  Branches          ?        0           
=========================================
  Hits              ?    45074           
  Misses            ?     4078           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.08% <100%> (?)` | |
| #single | `41.84% <100%> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20094?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/20094/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.64% <100%> (ø)` | |

------

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 @jreback required changes done, please review :grinning:  done!