Documentation: Fixed a typo in bootconfig.rst

Fixed a typo in the word 'concatenated'.

Signed-off-by: Kushagra Verma <kushagra765@outlook.com>
Link: https://lore.kernel.org/r/HK0PR01MB2801DA243B726141A97FE92EF8179@HK0PR01MB2801.apcprd01.prod.exchangelabs.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>