Update copy for revealing last hint
Update copy for revealing last hint

Summary:
In the exercise hint copy experiment, we're trying out different language around the hints feature. Another change we'd like to add is referring to the last hint as "the last step" instead of "another step".

I also updated the {x}/{y} ratio shown in the link for revealing another hint to refer to {the number of the next hint}/{the number of hints total} instead of {the number of hints currently visible}/{the number of hints total}.

Before:
{F711025}

After:
{F711026}

Asana task: https://app.asana.com/0/133265628488563/345270002501931

Test Plan:
Run `make serve` in your terminal,
run `make build` in another tab in your terminal,
navigate to `http://localhost:8080/math/precalculus/trig-equations-and-identities-precalc/inverse-trig-functions-precalc/e/inverse_trig_functions` in your browser,
reveal all the hints,
and verify that that last time you revealed a hint, the link copy said "Show the last hint (2/2)".

Reviewers: charlie, riley

Subscribers: jared

Differential Revision: https://phabricator.khanacademy.org/D35832

learningequality/perseus,ariabuckles/perseus,ariabuckles/perseus,ariabuckles/perseus,ariabuckles/perseus,learningequality/perseus