Fix Sphinx directive name

During the build of source code reference of Ceilometer Sphinx reported
the following error: Unknown directive type "code". The correct directive
is '.. code-block::' instead of '.. code::'. This was fixed in the
session.py. The other occurences of this directive were correct.

Change-Id: I7876d0c9e9614a688eb14d9da57bbdbf916a119a
Merge "Fix Sphinx directive name"