Correct Network.f property return type-hint
Correct Network.f property return type-hint

This is to address the issue I opened here: https://github.com/scikit-rf/scikit-rf/issues/611
jhillairet/scikit-rf,jhillairet/scikit-rf,jhillairet/scikit-rf,scikit-rf/scikit-rf,scikit-rf/scikit-rf