Update L3 advanced section for bug 1197026

The suggestion that subnet and network delete will remove the router interface, is incorrect. The rest of the instructions are correct.

The line should read:
  Then remove the interfaces from the router:

Change-Id: I3a85fb75a5a55023005c5f8ba564e60cb6a3fa60
Merge "Update L3 advanced section for bug 1197026"