Update NOTICE.txt
Update NOTICE.txt

- Minor copyright year updates

mattermost/desktop,yuya-oc/electron-mattermost,mattermost/desktop,yuya-oc/desktop,yuya-oc/electron-mattermost,yuya-oc/electron-mattermost,mattermost/desktop,yuya-oc/desktop,mattermost/desktop,yuya-oc/desktop,mattermost/desktop