docs(README): update git url to reflect new ownership
docs(README): update git url to reflect new ownership

The VRTK repo is now on the ExtendRealityLtd organisation so the
repository link has been updated to reflect this change.

thestonefox/VRTK,gomez-addams/SteamVR_Unity_Toolkit,thestonefox/SteamVR_Unity_Toolkit