FIX: Use fixed javasphinx

src/test/docker/Dockerfile

@@ -16,7 +16,4 @@ RUN pip3 install sphinx javasphinx recommonmark sphinx-rtd-theme sphinxcontrib-p
 # Alternative, if javasphinx fails because python3-sphinx is too recent:
 # (`_l` not found):
 #
-# git clone git@github.com:simgrid/javasphinx.git
-# cd javasphinx
-# git checkout 659209069603a
-# pip3 install .
+RUN git clone git@github.com:simgrid/javasphinx.git && cd javasphinx && git checkout 659209069603a && pip3 install .