Fix epydoc raises L{...} usage

For epydoc raise statement, it already expects an exception which will
be hyperlinked; if passing an L{…} construct, it will instead break
the formatting (the @raise will not be recognised, but merged into the
previous paragraph).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>