Adjusts statistics translation keys

fixes CNVS-14933

test plan:
  - Make sure it works for Transifex

Change-Id: I6598e5dc5854fbcea70caf77f2373817e17e3494
Reviewed-on: https://gerrit.instructure.com/39583
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Cosme Salazar <cosme@instructure.com>

app/views/courses/statistics.html.erb

@@ -82,8 +82,8 @@
     <table>
       <thead><tr>
         <th><%= t('submission_type', %{Submission Type}) %></th>
-        <th><%= t('num_assignments', %{# Assignments}) %></th>
-        <th><%= t('num_submissions', %{# Submissions}) %></th>
+        <th><%= t('num_of_assignments', %{# Assignments}) %></th>
+        <th><%= t('num_of_submissions', %{# Submissions}) %></th>
       </tr>
       </thead>
       <tfoot>