[Docs] Fix a typo in index.md

[MyeongKim]

Why are these changes needed?

Fix a small typo in Getting Started Guide (how to user Ray Core -> how to use Ray Core)

Related issue number

Checks

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[dmatrix]

Good catch. Thanks for the fix @MyeongKim

[pcmoritz]

Thanks a lot for the fix :)

[MyeongKim]

Thank you! @dmatrix @pcmoritz