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: 51e46c7 ("docs, parallelism: Rearrange how jobserver reservations are made")
Cc: [email protected]
Signed-off-by: Kees Cook <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jonathan Corbet <[email protected]>

Author: kees

Documentation/Makefile

@@ -76,7 +76,7 @@ quiet_cmd_sphinx = SPHINX  $@ --> file://$(abspath $(BUILDDIR)/$3/$4)
 	PYTHONDONTWRITEBYTECODE=1 \
 	BUILDDIR=$(abspath $(BUILDDIR)) SPHINX_CONF=$(abspath $(srctree)/$(src)/$5/$(SPHINX_CONF)) \
 	$(PYTHON3) $(srctree)/scripts/jobserver-exec \
-	$(SHELL) $(srctree)/Documentation/sphinx/parallel-wrapper.sh \
+	$(CONFIG_SHELL) $(srctree)/Documentation/sphinx/parallel-wrapper.sh \
 	$(SPHINXBUILD) \
 	-b $2 \
 	-c $(abspath $(srctree)/$(src)) \