docs: fix README item key casing

[thehanslevi]

Summary

Fixes the README example to use items[i].Key, matching the Item struct field declared earlier in the same example.

Validation

• Confirmed the README defines type Item struct { Key string } and already uses item.Key when adding keys to the builder.
• Confirmed there are no open PRs overlapping this README typo.
• Ran git diff --check.

I could not run the Go test suite locally because this environment does not have a go binary installed.