DOC: fix indention in CONTRIBUTING

CONTRIBUTING.rst

@@ -20,13 +20,13 @@ Development Installation
 Instead of pip-installing the latest release from PyPI_,
 you should get the newest development version from Github_::
 
-   git clone https://github.com/audeering/audmath/
-   cd audmath
-   # Create virtual environment for this project
-   # e.g.
-   # virtualenv --python="python3"  $HOME/.envs/audmath
-   # source $HOME/.envs/audmath/bin/activate
-   pip install -r requirements.txt
+    git clone https://github.com/audeering/audmath/
+    cd audmath
+    # Create virtual environment for this project
+    # e.g.
+    # virtualenv --python="python3"  $HOME/.envs/audmath
+    # source $HOME/.envs/audmath/bin/activate
+    pip install -r requirements.txt
 
 
 This way,
@@ -83,18 +83,18 @@ 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 docs/requirements.txt
+    pip install -r docs/requirements.txt
 
 To create the HTML pages, use::
 
-   python -m sphinx docs/ build/sphinx/html -b html
+    python -m sphinx docs/ build/sphinx/html -b html
 
 The generated files will be available
 in the directory :file:`build/sphinx/html/`.
 
 It is also possible to automatically check if all links are still valid::
 
-   python -m sphinx docs/ build/sphinx/html -b linkcheck
+    python -m sphinx docs/ build/sphinx/html -b linkcheck
 
 .. _Sphinx: http://sphinx-doc.org
 
@@ -105,11 +105,11 @@ Running the Tests
 You'll need pytest_ for that.
 It can be installed with::
 
-   pip install -r tests/requirements.txt
+    pip install -r tests/requirements.txt
 
 To execute the tests, simply run::
 
-   python -m pytest
+    python -m pytest
 
 .. _pytest: https://pytest.org