Fixed pluralization of "X item(s) selected"

For New Files Toolbar component

fixes CNVS-16837

Test Plan:
* Enable Better File Browsing
* Go to the Files page for a course
* Select one item in the file pane
* The text in the toolbar should read: "1 item selected"
* Select multiple items in the file pane
* The text in the tooldbar should read "`X` items selected"
  where X is the number of files you have selected

Change-Id: I1ac5dec632197df0826f79b7f3e09cf3a14e0f33
Reviewed-on: https://gerrit.instructure.com/44299
Reviewed-by: Ryan Shaw <ryan@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Dan Minkevitch <dan@instructure.com>
QA-Review: Jahnavi Yetukuri <jyetukuri@instructure.com>