Fix ReST formatting of pandas.read_excel usecols
Fix some reStructuredText issues in *usecols* parameter of read_excel

- bulletlists must start with a space
- the deprecated/versionadded directives do only apply to the respective bullet point an thus need to be indented.

Comapre http://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.read_excel.html for the current formatting issues.

Thanks for the PR. Was this causing an issue in rendering? Quick glance at production docs look OK # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25801?src=pr&el=h1) Report
> Merging [#25801](https://codecov.io/gh/pandas-dev/pandas/pull/25801?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/85c3f82dff73ede6d5d961611518e007c8ba0098?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/25801/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25801?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #25801   +/-   ##
=======================================
  Coverage   91.27%   91.27%           
=======================================
  Files         173      173           
  Lines       53002    53002           
=======================================
  Hits        48375    48375           
  Misses       4627     4627
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.83% <ø> (ø)` | :arrow_up: |
| #single | `41.77% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25801?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/excel/\_base.py](https://codecov.io/gh/pandas-dev/pandas/pull/25801/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2V4Y2VsL19iYXNlLnB5) | `92.48% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25801?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/25801?src=pr&el=footer). Last update [85c3f82...ee7e6ee](https://codecov.io/gh/pandas-dev/pandas/pull/25801?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25801?src=pr&el=h1) Report
> Merging [#25801](https://codecov.io/gh/pandas-dev/pandas/pull/25801?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/85c3f82dff73ede6d5d961611518e007c8ba0098?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/25801/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25801?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #25801   +/-   ##
=======================================
  Coverage   91.27%   91.27%           
=======================================
  Files         173      173           
  Lines       53002    53002           
=======================================
  Hits        48375    48375           
  Misses       4627     4627
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.83% <ø> (ø)` | :arrow_up: |
| #single | `41.77% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25801?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/excel/\_base.py](https://codecov.io/gh/pandas-dev/pandas/pull/25801/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2V4Y2VsL19iYXNlLnB5) | `92.48% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25801?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/25801?src=pr&el=footer). Last update [85c3f82...ee7e6ee](https://codecov.io/gh/pandas-dev/pandas/pull/25801?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @WillAyd:
1. Two bullet points are lost in the text follow.
2. The deprecated/versionadded notes should be indented to align with the text of their respective bullet point.

![grafik](https://user-images.githubusercontent.com/2836374/54710345-c0bec600-4b47-11e9-8536-cb7ae7e97616.png)
 Makes sense - thanks @timhoffm 