docs: Fix empty parallelism argument

When there was no parallelism (no top-level -j arg and a pre-1.7
sphinx-build), the argument passed would be empty ("") instead of just
being missing, which would (understandably) badly confuse sphinx-build.
Fix this by removing the quotes.

Reported-by: Rafael J. Wysocki <rafael@kernel.org>
Fixes: 51e46c7a ("docs, parallelism: Rearrange how jobserver reservations are made")
Cc: stable@vger.kernel.org  # v5.5 only
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>