docs: admin-guide/index.rst: Add pstore-blk.rst

Fix the following warning:

Documentation/admin-guide/pstore-blk.rst:
WARNING: document isn't included in any toctree

By adding 'pstore-blk.rst' to the index

Signed-off-by: Daniel W. S. Almeida <dwlsalmeida@gmail.com>
Link: https://lore.kernel.org/r/20200718165107.625847-9-dwlsalmeida@gmail.com


Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>