[MERGE] Correct incorrect wording in revisionspec help

[Ian Clatworthy]

Matthew D. Fuller wrote:
> SSIA.  The information for how ranges in diff work is incorrect.

bb:approve

> I would also propose this for 1.11.  It's tiny and obviously incorrect
> (the fix may be wrong too, but at least we know it's wrong before  :)

That's Martin's call as he's the Release Manager. I'm fine with it
going in.

Ian C.