DOCS:Update index.md

[Donal-Power]

fixing typo to resolve DOCF-4184

Description

Fixed user-reported bug in documentation

Changes made

fixed typo to resolve DOCF-4184

Notes

Please write down any additional notes, remove the section if not applicable.

Checklist

Before review:

• [ ] Changelog entry added.
  • Explains the change in Changed, Fixed, Added sections.
  • For API change contains an example snippet and/or migration example.
  • FogBugz ticket attached, example ([case %number%](https://issuetracker.unity3d.com/issues/...)).
  • FogBugz is marked as "Resolved" with next release version correctly set.
• [ ] Tests added/changed, if applicable.
  • Functional tests Area_CanDoX, Area_CanDoX_EvenIfYIsTheCase, Area_WhenIDoX_AndYHappens_ThisIsTheResult.
  • Performance tests.
  • Integration tests.
• [ ] Docs for new/changed API's.
  • Xmldoc cross references are set correctly.
  • Added explanation how the API works.
  • Usage code examples added.
  • The manual is updated, if needed.

During merge:

• [ ] Commit message for squash-merge is prefixed with one of the list:
  • NEW: ___.
  • FIX: ___.
  • DOCS: ___.
  • CHANGE: ___.
  • RELEASE: 1.1.0-preview.3.

[unity-cla-assistant]

All committers have signed the CLA.

[ekcoh]

@duckets Since this seems to be coming from a fork I suggest you incorporate this typo correction into one of your pending documentation branches instead and close this PR without merge. This will reduce overhead of forward/backporting since this change is very small.

[ekcoh]

@Donal-Power This PR should be closed right since superceeded by https://github.com/Unity-Technologies/InputSystem/pull/1952?

[ekcoh]

I am closing this PR now, feel free to reopen if necessary but I believe this have been superseded by #1952.