DOC: Reword doc for filepath_or_buffer in read_csv
Shortens dtype description following colon and moves it to a more expanded parameter description.

Closes #22057.

cc @WillAyd 
Hello @gfyoung! Thanks for updating the PR.



Cheers ! There are no PEP8 issues in this Pull Request. :beers: 

##### Comment last updated on July 26, 2018 at 08:37 Hours UTC # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22058?src=pr&el=h1) Report
> Merging [#22058](https://codecov.io/gh/pandas-dev/pandas/pull/22058?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/2c7c79779f1ef353d02446504d3b5f5c5cc68759?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/22058/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/22058?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #22058   +/-   ##
=======================================
  Coverage   92.02%   92.02%           
=======================================
  Files         170      170           
  Lines       50710    50710           
=======================================
  Hits        46664    46664           
  Misses       4046     4046
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.42% <ø> (ø)` | :arrow_up: |
| #single | `42.31% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/22058?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/parsers.py](https://codecov.io/gh/pandas-dev/pandas/pull/22058/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3BhcnNlcnMucHk=) | `95.46% <ø> (ø)` | :arrow_up: |

------

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> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22058?src=pr&el=footer). Last update [2c7c797...92cc578](https://codecov.io/gh/pandas-dev/pandas/pull/22058?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jreback @jorisvandenbossche : Made the requested changes, and all is green.  PTAL. Thanks!