Documentation/sphinx: fix make SPHINXDIRS="dirs" pdfdocs for more than one dir

Add missing semicolon to fix pdf build with more than one SPHINXDIRS
directory specified. For example make SPHINXDIRS="gpu media" pdfdocs.

Fixes: cd21379b ("doc-rst: generic way to build PDF of sub-folders")
Signed-off-by: Markus Heiser <markus.heiser@darmarIT.de>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>