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

Testing

None - Docs-only change

Risk

None - Docs-only change

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.
  • JIRA ticket linked, example (case %%). If it is a private issue, just add the case ID without a link.
  • Jira port for the next release set as "Resolved".
• [ ] 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.

After merge:

• [ ] Create forward/backward port if needed. If you are blocked from creating a forward port now please add a task to ISX-1444.