fix(CONTRIBUTING): change 72 lines to 72 characters
fix(CONTRIBUTING): change 72 lines to 72 characters

Commit messages are limited to 72 characters not lines.
Fulby/VRTK,thestonefox/SteamVR_Unity_Toolkit,gomez-addams/SteamVR_Unity_Toolkit,thestonefox/VRTK,adamjweaver/VRTK,virror/VRTK,red-horizon/VRTK,gpvigano/SteamVR_Unity_Toolkit,pargee/SteamVR_Unity_Toolkit,TMaloteaux/SteamVR_Unity_Toolkit,Innoactive/IA-unity-VR-toolkit-VRTK,gpvigano/VRTK,mattboy64/SteamVR_Unity_Toolkit