Fixed typo.

src/i18n/openfire_i18n_en.properties

@@ -2614,7 +2614,7 @@ system.email.info=Use the form below to set the host and port of your email serv
 		on your mail server. Note, if you choose to enable mail debugging the debug output will be written \
 		to your appserver's standard out log.
 system.email.update_success=SMTP settings updated successfully.
-system.email.update_failure=An error occured. Please verify that you have filled out all required fields \
+system.email.update_failure=An error occurred. Please verify that you have filled out all required fields \
         correctly and try again.
 system.email.name=SMTP Settings
 system.email.mail_host=Mail Host