Update README.md

Author: benpiper

elb/README.md

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-[lab-setup.ps1](lab-setup.ps1) - Refer to lab-setup.md for instructions
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+Course link: [AWS Networking Deep Dive: Elastic Load Balancing (ELB)](https://pluralsight.pxf.io/6bXjBK)
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+[lab-setup.ps1](lab-setup.ps1) - Refer to lab-setup.md for instructions