Clarify debugging instructions in README

[benjaminapetersen]

The line:

If you encouter any issues with the reporter, e.g. video files 
are not created, turn on debugging by settings the DEBUG 
environment to protractor-video-reporter.

Is somewhat unclear. It implies you would just DEBUG=protractor-video-reporter grunt protractor but I don't think that is correct.

[chrismwendt]

@benjaminapetersen Why don't you think that's correct? env DEBUG=protractor-video-reporter grunt... works for me.

[benjaminapetersen]

i don't believe it worked for me, but, that was Oct last year. I cant say that I recall at this point.