Fixed relative url for jsonrpc endpoint

Using '/jsonrpc' can cause problems if you have a proxy infront of xbmc and it already has a relative URL.

Simply removing the '/' form the front of the jsonrpc url will fix it (and does not break non-proxied access). 
The problem is if you already have a relative url (https://mydomain.com/xbmcbox), then /jsonrpc will try to hit https://mydomain.com/jsonrpc (gets 404). If you change it to 'jsonrpc', then it will work correctly and hit https://mydomain.com/xbmcbox/jsonrpc