docs: Update init.md, add missed '@'

[Jongkeun]

Summary

Correct the missing '@' from the package name in the command required to create a project in the init.md document.

Test Plan

No need.

Checklist

• Documentation is up to date.
• Follows commit message convention described in CONTRIBUTING.md.
• For functional changes, my test plan has linked these CLI changes into a local react-native checkout (instructions).

[thymikee]

Oops! Thank you