docs-rst: fix a breakage when building PDF documents

changeset 606b9ac8 ("doc-rst: generic way to build only sphinx
sub-folders") accidentally broke PDF generation by adding an extra
")". Remove it.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>