Outdated Wrangler KV namespace creation command in README

**The README.md file contains an outdated command for creating a KV namespace using Wrangler:**

`npx wrangler kv:namespace create "WORKER_MAPPINGS"`

This syntax with colons (kv:namespace) is deprecated. Since Wrangler version 3.60.0, the correct syntax is:

`npx wrangler kv namespace create "WORKER_MAPPINGS"`

**Steps to Reproduce**
- Follow the getting started guide in README.md.
- Run the command in step 4.
- Encounter an "Unknown arguments" error because the command is invalid.

**Expected Behavior**
- The command should work without errors.

**Actual Behavior**
- Wrangler throws an error: [ERROR] Unknown arguments: kv:namespace, create, WORKER_MAPPINGS

**Environment**
- Wrangler version: 4.38.0 (and above)
- OS: macOS (and others)

**Fix**
- Update the command in README.md to use the new syntax.