docs: Correct formatting of title in admin-guide/index.rst

Adjust the title of "The Linux kernel user's and administrator's guide" to
adhere to the expected reStructuredText (rst) formatting, using double
equal signs for the main header.

Signed-off-by: Carlos Bilbao <carlos.bilbao@amd.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240109155643.3489369-2-carlos.bilbao@amd.com