Update crash_course.rst
Update crash_course.rst

reference to a inexistent `myVideo` variable.
DevinGeo/moviepy,LunarLanding/moviepy,deniszgonjanin/moviepy,bertyhell/moviepy,carlodri/moviepy,mcanthony/moviepy,ssteo/moviepy,TheNathanBlack/moviepy,benroeder/moviepy,DevinGeo/moviepy,Zulko/moviepy,Vicaris/ModPro,kerstin/moviepy,misterbisson/moviepy