Fix common misspellings

Fixes generated by 'codespell' and manually reviewed.

Signed-off-by: Lucas De Marchi <lucas.demarchi@profusion.mobi>
diff --git a/kernel/user-return-notifier.c b/kernel/user-return-notifier.c
index eb27fd3..92cb706 100644
--- a/kernel/user-return-notifier.c
+++ b/kernel/user-return-notifier.c
@@ -20,7 +20,7 @@
 
 /*
  * Removes a registered user return notifier.  Must be called from atomic
- * context, and from the same cpu registration occured in.
+ * context, and from the same cpu registration occurred in.
  */
 void user_return_notifier_unregister(struct user_return_notifier *urn)
 {