Fix readthedocs documentation

Author: mcbarton

docs/source/InstallationAndUsage.rst

@@ -161,7 +161,8 @@ To build and test Jupyter Lite with this kernel locally you can execute the foll
                        --contents notebooks/images/marie.png \
                        --contents notebooks/audio/audio.wav
 
-### xeus-cpp Jupyter Lite tests
+xeus-cpp Jupyter Lite tests
+========================
 
 It is possible to run test the xeus-cpp in Jupyter Lite deployment, using a Selenium based script and the nbdiff Jupyter tool.
 In order to install these dependencies execute the following (you will need the Jupyter Lite website running in the background when