cpu/cortexm_common: fix typo 'occured' -> 'occurred'
cpu/cortexm_common: fix typo 'occured' -> 'occurred'

authmillenon/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,authmillenon/RIOT,jasonatran/RIOT,miri64/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,miri64/RIOT,kaspar030/RIOT,authmillenon/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,OlegHahm/RIOT,kaspar030/RIOT,miri64/RIOT,authmillenon/RIOT,kYc0o/RIOT,OlegHahm/RIOT,authmillenon/RIOT,jasonatran/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,ant9000/RIOT,ant9000/RIOT,kYc0o/RIOT,kYc0o/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,ant9000/RIOT,kYc0o/RIOT,ant9000/RIOT,OlegHahm/RIOT,miri64/RIOT,authmillenon/RIOT,kaspar030/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,kYc0o/RIOT,jasonatran/RIOT,miri64/RIOT,jasonatran/RIOT,ant9000/RIOT