fix form label for voluntary_hours_per_week field
fix form label for voluntary_hours_per_week field

make it explicit that only integer/numbers can be entered, as no errors/warnings are shown when entering normal text

beanieboi/rgsoc-teams,ramonh/rgsoc-teams,ramonh/rgsoc-teams,rails-girls-summer-of-code/rgsoc-teams,ramonh/rgsoc-teams,beanieboi/rgsoc-teams,rails-girls-summer-of-code/rgsoc-teams,beanieboi/rgsoc-teams,michaelem/rgsoc-teams,TeamCheesy/rgsoc-teams,rails-girls-summer-of-code/rgsoc-teams,michaelem/rgsoc-teams,TeamCheesy/rgsoc-teams,TeamCheesy/rgsoc-teams,michaelem/rgsoc-teams