finatra: Update Link to Finagle HTTP Request
finatra: Update Link to Finagle HTTP Request

Problem / Solution

The link to Finagle HTTP's `Request` no longer
works on the Finatra User Guide's 'requests' page. Let's
fix it.

Differential Revision: https://phabricator.twitter.biz/D412209

twitter/finatra,twitter/finatra,twitter/finatra