[DOC] Renamed SarosNet -> XMPPConnectionService

This refactoring was performed before
(https://github.com/saros-project/saros/commit/6be0677).
This patch amends the architecture graphic of the network layer
accordingly.

Change-Id: Ifab5663759f8c66eaa54b5d9fccae1b0bb6f1904
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1463
Reviewed-by: Franz Zieris <zieris@inf.fu-berlin.de>
Tested-by: Franz Zieris <zieris@inf.fu-berlin.de>