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)