docs: fix outdated Wrangler KV namespace command

[thehanslevi]

Summary

• replace the deprecated wrangler kv:namespace create command in the README
• use the current wrangler kv namespace create syntax shown by current Wrangler help output

Why

The README currently documents a command form that current Wrangler rejects with Unknown argument: kv:namespace.

Validation

• confirmed README.md still uses npx wrangler kv:namespace create "WORKER_MAPPINGS"
• ran npx --yes wrangler@latest kv namespace create --help
• ran npx --yes wrangler@latest kv:namespace create --help and verified it fails with Unknown argument: kv:namespace

Closes #44.