docs: Fix "make help" suggestion for SPHINXDIR

Commit 9fc3a18a ("docs: remove extra conf.py files") broke the setting
of _SPHINXDIRS in Documentation/Makefile.  Let's just have it look for an
index.rst file instead.

Fixes: 9fc3a18a ("docs: remove extra conf.py files")
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/Makefile

@@ -13,7 +13,7 @@ endif
 SPHINXBUILD   = sphinx-build
 SPHINXOPTS    =
 SPHINXDIRS    = .
-_SPHINXDIRS   = $(patsubst $(srctree)/Documentation/%/conf.py,%,$(wildcard $(srctree)/Documentation/*/conf.py))
+_SPHINXDIRS   = $(patsubst $(srctree)/Documentation/%/index.rst,%,$(wildcard $(srctree)/Documentation/*/index.rst))
 SPHINX_CONF   = conf.py
 PAPER         =
 BUILDDIR      = $(obj)/output