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

Documentation/admin-guide/index.rst

@@ -41,6 +41,7 @@ problems and bugs in particular.
    init
    kdump/index
    perf/index
+   pstore-blk
 
 This is the beginning of a section with information of interest to
 application developers.  Documents covering various aspects of the kernel