Correct docs for AWS_VPC_K8S_CNI_RANDOMIZESNAT

[orirawlings]

Issue #, if available:
Fixes #380

Description of changes:

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[orirawlings]

Looks like the travis build failed because of a tcp timeout:

go: k8s.io/api@v0.0.0-20180712090710-2d6f90ab1293: unrecognized import path "k8s.io/api" (https fetch: Get https://k8s.io/api?go-get=1: dial tcp 35.201.71.162:443: i/o timeout)
...
go: error loading module requirements
make: *** [build-linux] Error 1
The command "GO111MODULE=on make build-linux" exited with 2.

Not related to the changes in the PR.

[mogren]

Thanks @orirawlings for the doc fix!