Rename legacy token for contribution id to match sched reminder tokens

Rename legacy token for contribution id to match sched reminder tokens

In this case it turns out

1) the token being advertised by scheduled reminders is more correct ('id')
2) the legacy method resolves either 'id' or 'contribution_id' and
3) changing the advertised token from 'contribution.contribution_id' to contribution.id
does not cause contribution_id to stop working

So in summary - the advertised token was working in 'send letter' but not
in scheduled reminders. And changing which is advertised doesn't stop
the other from working in send letter