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>