fix phrasing in external feed tray
fix phrasing in external feed tray

fixes COMMS-919

Test Plan:
- With SSA on
- go to announcements index page
- go to the external feeds tray
- open the tray
- notice the wording is changed next to the only
checkbox in the tray

Change-Id: Ic0d6de14a4920aa89219341e3c02ab5bc727732d
Reviewed-on: https://gerrit.instructure.com/142139
Reviewed-by: Aaron Kc Hsu <089edc94045de3d9f5f372fb17706d21afcc259f@instructure.com>
Tested-by: Jenkins
Product-Review: Steven Burnett <a0b450117c5d20a22d5c4d52bb8ae13adee003d8@instructure.com>
QA-Review: Steven Burnett <a0b450117c5d20a22d5c4d52bb8ae13adee003d8@instructure.com>

instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,sfu/canvas-lms