Documentation/admin-guide: tainted-kernels: Fix typo occured

Fix typo.
s/occured/occurred/

Signed-off-by: Naoki Hayama <naoki.hayama@lineo.co.jp>
Link: https://lore.kernel.org/r/20201012082441.5831-1-naoki.hayama@lineo.co.jp


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