Fix the bad value of EC2_HOME in the standard_instructions

Change EC2_HOME's value from 'export EC2_HOME="/usr/local/Cellar/ec2-api-tools/1.6.12.0/"'
to 'export EC2_HOME="/usr/local/Cellar/ec2-api-tools/1.6.12.0/libexec"'

Avoide the wrong value of EC_HOME caused by
024208466bfb4ffb479d48709caecc6a1e85ba4d