fix: "Back up" is two words when used as a verb (openedx#2706)

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"

Backports 70c19a3

Author: kdmccormick

src/header/messages.js

@@ -103,8 +103,8 @@ 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.links.optimizer': {
     id: 'header.links.optimizer',