chore(README): remove additional markup for common link
chore(README): remove additional markup for common link

The `SteamVR Plugin` link is a common link referenced at the bottom of 
the document, all references to that link don't need to include the full
link inline.
Frostlock/SteamVR_Unity_Toolkit,gpvigano/SteamVR_Unity_Toolkit,Blueteak/SteamVR_Unity_Toolkit,mbbmbbmm/SteamVR_Unity_Toolkit,thacio/SteamVR_Unity_Toolkit,thestonefox/VRTK,thestonefox/SteamVR_Unity_Toolkit,Innoactive/IA-unity-VR-toolkit-VRTK,mimminito/SteamVR_Unity_Toolkit,pargee/SteamVR_Unity_Toolkit,mattboy64/SteamVR_Unity_Toolkit,virror/VRTK,phr00t/SteamVR_Unity_Toolkit,Fulby/VRTK,gomez-addams/SteamVR_Unity_Toolkit,jcrombez/SteamVR_Unity_Toolkit,nmccarty/htvr,gpvigano/VRTK,adamjweaver/VRTK,bengolds/SteamVR_Unity_Toolkit,red-horizon/VRTK,TMaloteaux/SteamVR_Unity_Toolkit