Since Sphinx 2.3.0 backquotes are replaced to \textasciigrave{} during
building latexpdf. Using this element in a \sphinxhref{} breaks the
build. In order to avoid this error backquotes must not be used in
external hyperlinks.

Signed-off-by: Imre Kis <imre.kis@arm.com>
Change-Id: Ie3cf454427e3d5a7b7f9829b42be45aebda7f0dd