Fixing typo.

README.md

 * ARKit Face Tracking ([documentation](https://docs.unity3d.com/Packages/com.unity.xr.arkit-face-tracking@4.0/manual/index.html))
 * ARFoundation ([documentation](https://docs.unity3d.com/Packages/com.unity.xr.arfoundation@4.0/manual/index.html))
 
-## Why version should I use?
+## What version should I use?
 
 A Unity package is either "Preview" or "Verified". The latest version of ARFoundation is usually marked as preview and may include experimental or unstable features. A "verified" package is developed targeting a specific version of Unity (though it may work with earlier version as well). All packages verified for the same version of Unity are known to work well together.