DOC: update the pandas.Series.str.slice_replace 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>`
- [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.Series.str.slice_replace)  #################
################################################################################

Replace a sliced string.

Replace a slice of each string in the Series/Index with another
string.

Parameters
----------
start : int or None
    Left edge index.
stop : int or None
    Right edge index.
repl : str or None
    String for replacement.

Returns
-------
replaced : Series/Index of objects

Examples
--------
>>> s = pd.Series(['This is a Test 1', 'This is a Test 2'])
>>> s
0    This is a Test 1
1    This is a Test 2
dtype: object
>>> s = s.str.slice_replace(8, 14, 'an Example')
>>> s
0    This is an Example 1
1    This is an Example 2
dtype: object

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

Errors found:
	See Also section not found

```
Updated if you want to take a look.

I'm not sure how, but apparently changing the default `repl` to `''` broke tests.  https://github.com/pandas-dev/pandas/pull/20273/commits/7a0cf0c93ad4778c78a6c7a477f9309cc6e13403 reverted that to be how it was before. Will let the tests run on this before merging. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20273?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@ea8ac51`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20273/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/20273?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #20273   +/-   ##
=========================================
  Coverage          ?   91.77%           
=========================================
  Files             ?      152           
  Lines             ?    49178           
  Branches          ?        0           
=========================================
  Hits              ?    45133           
  Misses            ?     4045           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.16% <ø> (?)` | |
| #single | `41.84% <ø> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20273?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/strings.py](https://codecov.io/gh/pandas-dev/pandas/pull/20273/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3RyaW5ncy5weQ==) | `98.32% <ø> (ø)` | |

------

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> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20273?src=pr&el=footer). Last update [ea8ac51...5263522](https://codecov.io/gh/pandas-dev/pandas/pull/20273?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
