docs: fix code blocks (#506)

Previously: https://i.imgur.com/XWFhG7a.png

Author: MicaelJarniac

docs/index.rst

@@ -28,14 +28,14 @@ your project, for example ``setup.py``::
    )
 
 Python Semantic Release is configured using ``setup.cfg`` or ``pyproject.toml``.
-Set :ref:`config-version_variable` to the location of your version variable inside any Python file::
+Set :ref:`config-version_variable` to the location of your version variable inside any Python file:
 
-``setup.cfg``:
+``setup.cfg``::
 
    [semantic_release]
    version_variable = setup.py:__version__
 
-``pyproject.toml``:
+``pyproject.toml``::
 
    [tool.semantic_release]
    version_variable = "setup.py:__version__"