Docs: Fix link in Lambda Extensions

[bessb]

The supported runtimes link is not rendering properly due to incorrect Markdown.

[vercel[bot]]

@bessb is attempting to deploy a commit to the HashiCorp Team on Vercel.

A member of the Team first needs to authorize it.

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[tjperry07]

@bessb Review the contributer guidelines and I can get this merged for you.

[bessb]

Sorry, didn't realize I'd need to sign a CLA for a doc change. I'll need to leave it open for someone else to fix.