Set font correctly in canvas example.

This fixes the following warning:

Context2D: Missing or misplaced font family in font string (it must come after the font size).

Task-number: QTBUG-38103
Change-Id: I0902e6e095fb76cdc8619229dccf2601007c3216
Reviewed-by: Jeff Tranter <jtranter@ics.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>