Documentation: isdn: correct spelling

Correct spelling problems for Documentation/isdn/ as reported
by codespell.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Karsten Keil <isdn@linux-pingi.de>
Cc: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20230209071400.31476-9-rdunlap@infradead.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>