fix bug in routes for cancel membership request

'cancel request' button on group page was not working.

this was due to a difference between a link_to and a route 
- link_to was trying to send a DELETE request
- route was for PUT requests.

investigating the Memberships Controller, I found the cancel_request action, which is calling @membership.destroy 

so i changed the routes to match the link_to (both are DELETE now)