Remove amount field from campaign leaderboard sample response

The campaign leaderboard (http://developer.everydayhero.com/leaderboards/#view-a-campaign-leaderboard) no longer returns the amount. This doco change is in relation to the following code change:

https://github.com/everydayhero/fundraiser/commit/a57a15a9a239cd882c53db46667119abb9e408de#app/serializers/campaign_leaderboard_serializer.rb