[DOC] Add list mark to keep select_columns's example same style

[Zeroto521]

PR Description

Please describe the changes proposed in the pull request:

As title.
When I read select_columns's doc, I found the 3329 line missing the list mark.

pyjanitor/janitor/functions.py

Lines 3322 to 3329 in b806e29

	- Select by string::
	df.select_columns("id")
	id
	0 0
	1 1
	Select via shell-like glob strings (*) is possible::

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Relevant Reviewers

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• @ericmjl

[Zeroto521]

Should 3290 line change to .. code-block:: python ?
But this doesn't seem any effect on HTML.

pyjanitor/janitor/functions.py

Lines 3287 to 3292 in b806e29

	Examples
	--------
	::
	import pandas as pd

[codecov]

Codecov Report

Merging #888 (11b7186) into dev (b806e29) will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##              dev     #888      +/-   ##
==========================================
+ Coverage   95.62%   95.66%   +0.04%     
==========================================
  Files          19       19              
  Lines        2444     2444              
==========================================
+ Hits         2337     2338       +1     
+ Misses        107      106       -1     

[ericmjl]

@Zeroto521 thank you for handling this!

[ericmjl]

@Zeroto521 thank you very much!