Correct a zero-based month tooltip on burnup charts

Summary: See PHI1017. This is a trivial fix even though these burnups are headed toward a grisly fate.

Test Plan: Moused over some January datapoints, saw "1" instead of "0".

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D19967