stop, start and destroy require the EC2_REGION

i.e  
./spark-ec2 --region=us-west-1 stop yourclustername
the format <EC2_REGION>  UPPERCASE_UNDERSCORE was used throughout the rest of the documentation earlier.
This change was to indicate the python script would not work with "stop" unless region was passed as an argument.
It would need to be a global replace if we wanted to reflect the same approach earlier in the document.
Other UPPERCASE_SCORE text already in the docs include things like:
<INSTANCE_TYPE>
<EC2_ZONE>
etc..

<cluster-name> is lowercase in the rest of the doc.