Docs on Mutations: Fixed issue with relay.NodeInput not existing, imported NodeInput from strawberry_django instead

[thepapermen]

Description

Docs on Mutations: had mentioned a relay.NodeInput usage which doesn't exist on relay and actually is a part of strawberry_django

Types of Changes

• [ ] Core
• [] Bugfix
• [ ] New feature
• [ ] Enhancement/optimization
• [x] Documentation

Issues Fixed or Closed by This PR

• Fix #354

Checklist

• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [ ] I have tested the changes and verified that they work and don't break anything (as well as I can manage).