settings_org: Add missing closing square bracket.
settings_org: Add missing closing square bracket.

Due to closing bracket not being present, discard
button of "Theme" subsection of default user settings
resulted in error.

zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip