Unify binding ref in R2 docs

[lkolbly]

Summary

The docs say to create a binding named "MY_BINDING", this updates the code to match.

Documentation checklist

• The change adheres to the documentation style guide.

[cloudflare-docs-bot]

Review

✅ No issues found in commit 8e65099.

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[helloimalastair]

A few small changes, otherwise lgtm

[helloimalastair]

LGTM