MarkdownBear: Replace `markdown_horizontal_rule`

[yash-nisar]

Replace markdown_horizontal_rule in the docstring by
horizontal_rule which is used create_arguments(...).

Closes #1907

For short term contributors: we understand that getting your commits well
defined like we require is a hard task and takes some learning. If you
look to help without wanting to contribute long term there's no need
for you to learn this. Just drop us a message and we'll take care of brushing
up your stuff for merge!

Checklist

• I read the commit guidelines and I've followed
  them.
• I ran coala over my code locally. (All commits have to pass
  individually. It is not sufficient to have "fixup commits" on your PR,
  our bot will still report the issues for the previous commit.) You will
  likely receive a lot of bot comments and build failures if coala does not
  pass on every single commit!

After you submit your pull request, DO NOT click the 'Update Branch' button.
When asked for a rebase, consult coala.io/rebase
instead.

Please consider helping us by reviewing other peoples pull requests as well:

• pick up any PR at https://coala.io/review
• review it (check https://coala.io/reviewing for more info)
• if you are sure that it needs work, use cobot mark wip <URL> to get it out
  of the review queue.

The more you review, the more your score will grow at coala.io and we will
review your PRs faster!

[yash-nisar]

CC @Makman2 , a quick one 😉

[yash-nisar]

CC @Makman2 , a quick one 😉

[Makman2]

ack e043a2e

[Makman2]

ack e043a2e

[yash-nisar]

reack e2cf115

[yash-nisar]

reack e2cf115

[Makman2]

@rultor merge

[Makman2]

@rultor merge

[rultor]

@rultor merge

@Makman2 OK, I'll try to merge now. You can check the progress of the merge here

[rultor]

@rultor merge

@Makman2 OK, I'll try to merge now. You can check the progress of the merge here

[rultor]

@rultor merge

@Makman2 Done! FYI, the full log is here (took me 2min)

[rultor]

@rultor merge

@Makman2 Done! FYI, the full log is here (took me 2min)