Updated INSTALL

Okay, third time should be the charm on this, I did double check when creating this pull request, there is one commit, only one file is changed. The text is 80 columns with the exception of one line in the Mac section, however that existed prior to my editing of the file, so I will leave it as is. When compiling ffmpeg, I only include the modules that can currently be utilized by OBS Studio. All lines are indented using spaces, and the indent is set to 2. I can confirm that this was checked using Sublime Text. If this request is rejected as well, I am probably going to turn to the wiki, I am more of a front-end/documentation person anyway so I tend not to worry too much about formatting.