fix: "Back up" is two words when used as a verb (#2696)

There is a new menu item "Backup to local archive". Backup is the correct
spelling when using it as a noun or adjective, but the menu item uses as a
verb, so it should be two words, back up, i.e. "Back up to local archive"

src/header/hooks.test.ts

@@ -174,7 +174,7 @@ describe('header utils', () => {
       const items = renderHook(() => useLibraryToolsMenuItems('course-123')).result.current;
       expect(items).toContainEqual({
         href: '/library/course-123/backup',
-        title: 'Backup to local archive',
+        title: 'Back up to local archive',
       });
       expect(items).toContainEqual({ href: '/library/course-123/import', title: 'Import' });
     });

src/header/messages.ts

@@ -108,8 +108,8 @@ const messages = defineMessages({
   },
   'header.links.exportLibrary': {
     id: 'header.links.exportLibrary',
-    defaultMessage: 'Backup to local archive',
-    description: 'Link to Studio Backup Library page',
+    defaultMessage: 'Back up to local archive',
+    description: 'Link to Studio Library Backup page',
   },
   'header.menu.teamAccess': {
     id: 'header.links.teamAccess',