Add missing period to get_dummies docs
Newline characters are stripped from the docstring when formatting parameter documentation. Because of this, the docs for the `prefix` parameter of [`pandas.get_dummies`](https://pandas.pydata.org/pandas-docs/stable/generated/pandas.get_dummies.html) look like a single run-on sentence. Added a period to the end of the first sentence to fix this.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21277?src=pr&el=h1) Report
> Merging [#21277](https://codecov.io/gh/pandas-dev/pandas/pull/21277?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/9af5fee41ddb2b13f2dd23792c3bc537795a3a63?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21277/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/21277?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #21277   +/-   ##
=======================================
  Coverage   91.84%   91.84%           
=======================================
  Files         153      153           
  Lines       49543    49543           
=======================================
  Hits        45504    45504           
  Misses       4039     4039
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.24% <ø> (ø)` | :arrow_up: |
| #single | `41.87% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21277?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/reshape/reshape.py](https://codecov.io/gh/pandas-dev/pandas/pull/21277/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9yZXNoYXBlLnB5) | `99.78% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21277?src=pr&el=continue).
> **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/21277?src=pr&el=footer). Last update [9af5fee...010058a](https://codecov.io/gh/pandas-dev/pandas/pull/21277?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!