Fix grammar errors in network admin guide

As noted by Dianne in
https://review.openstack.org/#/c/34576
there were a couple of places in the network admin guide
that could use a grammar fix. To avoid hassling the original
commiter, I submit them here for review.

Change-Id: I970642d9ff4c7f0cb247dca2c92438425e659400
Merge "Fix grammar errors in network admin guide"