What does this PR do?

Adds a checklist item to our contrib README file.

Also, while I'm here, fix a typo that has been haunting me.

Motivation

We didn't have a documented procedure to making sure our public docs stay up to date.

https://datadoghq.atlassian.net/browse/LANGPLAT-743

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.
• New code is free of linting errors. You can check this by running ./scripts/lint.sh locally.
• Add an appropriate team label so this PR gets put in the right place for the release notes.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild.

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