MarkdownBear: Replace `markdown_horizontal_rule`

[yash-nisar]

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

Closes #1907

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[yash-nisar]

CC @Makman2 , a quick one 😉

[Makman2]

ack e043a2e

[yash-nisar]

reack e2cf115

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