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 <[email protected]>
Signed-off-by: Jonathan Corbet <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: Zildj1an

Documentation/admin-guide/index.rst

@@ -1,3 +1,4 @@
+=================================================
 The Linux kernel user's and administrator's guide
 =================================================