doc: added missing default mapping `co` for `choose ours`

[kohane27]

Hello akinsho! Thank you for creating such an awesome tool. It makes resolving git conflict less error-prone:) Here's some changes I hope you'd find useful:

commit ceb5f9b115083033cbff662279085e618ec8c74d
    shellcheck: Use 'cd ... || exit' or 'cd ... || return' in case cd fails.

commit b4e3cd6a764b0b20d5fd4a57031d132b21c23603
    added missing default mapping `co` for `choose ours`

commit e79e7e9eb8b326833e64f7f6e0753eb2b2d0963b
    lua mapping list same order as default mapping

[akinsho]

Thanks @kohane27 👍🏿