DOC: Use "python3" and "python3 -m" in CONTRIBUTING

Author: mgeier

CONTRIBUTING.rst

@@ -6,12 +6,12 @@ please create an issue or a pull request at
 http://github.com/spatialaudio/python-sounddevice/.
 Contributions are always welcome!
 
-Instead of pip-installing the latest release from PyPI, you should get the newest
-development version from Github_::
+Instead of pip-installing the latest release from PyPI, you should get the
+newest development version from Github_::
 
    git clone --recursive https://github.com/spatialaudio/python-sounddevice.git
    cd python-sounddevice
-   python setup.py develop --user
+   python3 setup.py develop --user
 
 .. _Github: http://github.com/spatialaudio/python-sounddevice/
 
@@ -20,25 +20,25 @@ changes from the Github repository.
 
 If you prefer, you can also replace the last command with::
 
-   pip install --user -e .
+   python3 -m pip install --user -e .
 
 ... where ``-e`` stands for ``--editable``.
 
 If you used the ``--recursive`` option when cloning, the dynamic libraries for
 Mac OS X and Windows should be available.
 If not, you can get the submodule with::
 
-    git submodule update --init --recursive
+   git submodule update --init --recursive
 
 If you make changes to the documentation, you can re-create the HTML pages
 using Sphinx_.
 You can install it and a few other necessary packages with::
 
-   pip install -r doc/requirements.txt --user
+   python3 -m pip install -r doc/requirements.txt --user
 
 To create the HTML pages, use::
 
-   python setup.py build_sphinx
+   python3 setup.py build_sphinx
 
 The generated files will be available in the directory ``build/sphinx/html/``.