finagle/doc: Fix link in PartitionAwareClient documentation
finagle/doc: Fix link in PartitionAwareClient documentation

Problem/Solution

An incorrect prefix is getting rendered for the link to the
Thrift/ThriftMux client PartitioningParams link. Let's update
to a fully specified link to link correctly.

Differential Revision: https://phabricator.twitter.biz/D639286

twitter/finagle,twitter/finagle,twitter/finagle,twitter/finagle,twitter/finagle