Why are these changes needed?

Fix a small grammar typo on the docs landing page (Studio section):
“An web-based UI” → “A web-based UI.”

This improves readability and professionalism of the public documentation.

Related issue number

N/A (docs-only; trivial fix).
If maintainers prefer an issue, I can open one and link it.

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/.
  (Changed python/docs/src/index.md; no code behavior affected.)
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
  (Not applicable: docs-only change.)
• I've made sure all auto checks have passed.

Reference

Live page showing the typo (stable docs): https://microsoft.github.io/autogen/stable/index.html