change installing.rst for consistent virtualenv activate with installer

Installer prints:

To activate the Turbulenz env (required to use the tools),
invoke the env setup script with a command of the form:
    
  source /Users/eren/Turbulenz/SDK/0.25.1/start_env

Doc change for consistency
Merge pull request #7 from eguven/patch-1

change installing.rst for consistent virtualenv activate with installer