Fix markdown link error in docs

[MikeSchlosser16]

[google-cla[bot]]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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[bianpengyuan]

@MikeSchlosser16 Thanks for the PR! Please sign the google CLA.

[MikeSchlosser16]

@googlebot I signed it!

[bianpengyuan]

Sorry, but you'd also need to sign off the PR: https://github.com/proxy-wasm/proxy-wasm-cpp-sdk/pull/94/checks?check_run_id=2169438390