Issue with <code> elements in headings

[javiereguiluz]

This file: https://github.com/sonata-project/SonataAdminBundle/blob/3.x/docs/reference/dashboard.rst has things like these:

The ``Admin`` list block
------------------------

Configuring the ``Admin`` list
------------------------------

Using the ``Admin`` service declaration
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

These are wrongly processed in the TOC, which shows the links like this:

The contents of the heading are correctly displayed, but the "weird alphanumeric string" leaks to the permalink too:

[wouterj]

This is almost identical to doctrine/rst-parser#124 (but that was fixed in another place)

[nikophil]

thanks @wouterj for pointing this out :)

i've provided a fix on the parser doctrine/rst-parser#135

[javiereguiluz]

Closing as fixed. Thanks!