Mac os pipenv installation documentation bug fix (#711)

Fix mac os pipenv installation documentation

Co-authored-by: Tomás Sousa Pereira <[email protected]>

Author: tospe

README.rst

@@ -81,7 +81,7 @@ Virtualenv can avoid a lot of the QT / Python version issues
     pip3 install pipenv
     pipenv run pip install pyqt5==5.12.1 lxml
     pipenv run make qt5py3
-    python3 labelImg.py
+    pipenv run python3 labelImg.py
     [Optional] rm -rf build dist; python setup.py py2app -A;mv "dist/labelImg.app" /Applications
 
 Note: The Last command gives you a nice .app file with a new SVG Icon in your /Applications folder. You can consider using the script: build-tools/build-for-macos.sh