Fix ReST formatting of pandas.read_excel usecols

[timhoffm]

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.

[WillAyd]

Thanks for the PR. Was this causing an issue in rendering? Quick glance at production docs look OK

[codecov]

Codecov Report

Merging #25801 into master will not change coverage.
The diff coverage is n/a.

@@           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)	⬆️
#single	41.77% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/io/excel/_base.py	92.48% <ø> (ø)	⬆️

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[codecov]

Codecov Report

Merging #25801 into master will not change coverage.
The diff coverage is n/a.

@@           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)	⬆️
#single	41.77% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/io/excel/_base.py	92.48% <ø> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 85c3f82...ee7e6ee. Read the comment docs.

[timhoffm]

@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.

[WillAyd]

Makes sense - thanks @timhoffm