[TrivialFix] Fix AttributeError thrown in revision_documents
[TrivialFix] Fix AttributeError thrown in revision_documents

This is a self-evident fix, but errors.errors.UnknownSubstitutionError
is used instead of errors.UnknownSubstitutionError which is a
syntax mistake causing an AttributeError to get thrown; this
PS fixes that.

Change-Id: I0c988280fda8c68be63add0c581ebadb918c055a

att-comdev/deckhand,att-comdev/deckhand