Fixed the only sphinx warning in docs

Fixed the only sphinx warning in docs

We made previous attempt to get rid of all warnings, but it turned out
that gate does not execute tox's docs job but runs build_sphinx
directly.

The latter behaviour should be fixed, but while at it, we
should prepare neutron job to be executed in gate by cleaning up all
warnings.

Closes-Bug: #1466554

Change-Id: I8c265eae2175425568479116d1faef7d87fdcc02