Fix. [skip ci]

Author: mcmtroffaes

doc/usage.rst

@@ -626,13 +626,13 @@ If you do not want these back references,
 you can remove them using a css style sheet, as follows.
 First, add this to your ``conf.py``:
 
-.. code:: python
+.. code-block:: python
     html_static_path = ["_static"]
     html_css_files = ["custom.css"]
 
 Then, use this code for your ``_static/custom.css`` style sheet:
 
-.. code:: css
+.. code-block:: css
     .backrefs {
         display: none;
     }