docs: admin-guide: dell_rbu: Rework the title

- Mention the driver name, which is also used in sysfs (dell_rbu)
- Rewrite the title to be more concise

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/dell_rbu.rst

-=============================================================
-Usage of the new open sourced rbu (Remote BIOS Update) driver
-=============================================================
+=========================================
+Dell Remote BIOS Update driver (dell_rbu)
+=========================================
 
 Purpose
 =======