update doc link; add period

[CFly17]

Which issue(s) this PR fixes:
This PR updates a broken link (http://kubernetes.io/docs/admin/networking/) that returns 404 Not Found.

I considered linking to one of these docs:

• Design Proposal: Networking
• Concepts: Services/Networking

...but it seems Cluster Admin Concepts: Networking is most fitting as a simple doc that doesn't overemphasize Services, etc.

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[stmcginnis]

Looks like the appropriate link to me.

/lgtm