refactor: Rename remaining bugdown to markdown in markdown/__init__.py.
refactor: Rename remaining bugdown to markdown in markdown/__init__.py.

This commit is part of series of commits aimed at renaming bugdown to
markdown.

hackerkid/zulip,eeshangarg/zulip,hackerkid/zulip,showell/zulip,kou/zulip,kou/zulip,andersk/zulip,andersk/zulip,eeshangarg/zulip,zulip/zulip,kou/zulip,rht/zulip,showell/zulip,zulip/zulip,showell/zulip,rht/zulip,showell/zulip,rht/zulip,rht/zulip,hackerkid/zulip,showell/zulip,showell/zulip,rht/zulip,zulip/zulip,brainwane/zulip,andersk/zulip,kou/zulip,zulip/zulip,synicalsyntax/zulip,brainwane/zulip,punchagan/zulip,synicalsyntax/zulip,eeshangarg/zulip,andersk/zulip,synicalsyntax/zulip,punchagan/zulip,kou/zulip,showell/zulip,eeshangarg/zulip,kou/zulip,brainwane/zulip,zulip/zulip,eeshangarg/zulip,brainwane/zulip,hackerkid/zulip,synicalsyntax/zulip,hackerkid/zulip,rht/zulip,synicalsyntax/zulip,punchagan/zulip,eeshangarg/zulip,rht/zulip,hackerkid/zulip,punchagan/zulip,brainwane/zulip,punchagan/zulip,synicalsyntax/zulip,zulip/zulip,andersk/zulip,brainwane/zulip,synicalsyntax/zulip,hackerkid/zulip,andersk/zulip,punchagan/zulip,punchagan/zulip,zulip/zulip,andersk/zulip,kou/zulip,eeshangarg/zulip,brainwane/zulip