docs: Makefile: Use CONFIG_SHELL not SHELL

Fix think-o about which variable to find the Kbuild-configured shell.
This has accidentally worked due to most shells setting $SHELL by
default.

Fixes: 51e46c7a

 ("docs, parallelism: Rearrange how jobserver reservations are made")
Cc: stable@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20210617225808.3907377-1-keescook@chromium.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>