Fix venv directory typo in README

[LukeParky]

Overview

It looks like a simple mistake was made when developing the README.
The directory name pywps_flask_env was accidentally substituted with pywps_flask_dir

Related Issue / Discussion

#63

Additional Information

Contribution Agreement

(as per https://github.com/geopython/pywps/blob/master/CONTRIBUTING.rst#contributions-and-licensing)

• I'd like to contribute [feature X|bugfix Y|docs|something else] to PyWPS. I confirm that my contributions to PyWPS will be compatible with the PyWPS license guidelines at the time of contribution.
• I have already previously agreed to the PyWPS Contributions and Licensing Guidelines