Fix typo in python Dockerfile

Author: Aaron O'Mullan

stacks/python/Dockerfile

@@ -8,7 +8,7 @@ RUN virtualenv /opt/virtualenv; chown -R codebox:codebox /opt/virtualenv
 
 # Change path so we use the virtualenv's python interpreter
 # Install IPython
-RUN bash -c "su codebox bash -c 'source /opt/virtualenv/bin/activates && pip install ipython'"
+RUN bash -c "su codebox bash -c 'source /opt/virtualenv/bin/activate && pip install ipython'"
 
 # Default init command
 CMD su codebox /opt/codebox/init.sh