Update library.py
Update library.py

Fix for following issue:
tox -e doc produces the following error

Warning, treated as error:
/ws/hallmatt/git/topology_lib_tcpdump/.tox/doc/lib/python3.4/site-packages/topology_lib_tcpdump/library.py:docstring of topology_lib_tcpdump.library.tcpdump_capture_interface:8: WARNING: Field list ends without a blank line; unexpected unindent
HPENetworking/topology_lib_tcpdump