Improve error message in github integration to suggest a concrete next step.

[NorfairKing]

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Describe the user story

As a developer who's new to a repository and has never used atlantis before, I might see atlantis comment

the default workspace at path . is currently locked by another command that is running for this pull request.
Wait until the previous command is complete and try again

Describe the solution you'd like

Change that comment to say:

Wait until the previous command is complete and try again by commenting 'atlantis plan'.

Describe alternatives you've considered

STFU, not a big deal.