fix example code in the documentation

[omigamedev]

InsecureCredentials() is now InsecureChannelCredentials()

[thelinuxfoundation]

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[jtattermusch]

Please sign the CLA, otherwise I cannot merge.

[jtattermusch]

Also, if you could rebase, it would be nice (for cleaner commit history).

[omigamedev]

Just signed the CLA, but I'm not sure how to rebase.