fixes wording in document upload message

Changes "Please update this documents" to "Please update this document".

wagtail/documents/templates/wagtaildocs/multiple/add.html

@@ -56,7 +56,7 @@
                 </div>
             </div>
             <div class="right col9">
-                <p class="status-msg success">{% trans "Upload successful. Please update this documents with a more appropriate title, if necessary. You may also delete the document completely if the upload wasn't required." %}</p>
+                <p class="status-msg success">{% trans "Upload successful. Please update this document with a more appropriate title, if necessary. You may also delete the document completely if the upload wasn't required." %}</p>
                 <p class="status-msg failure">{% trans "Sorry, upload failed." %}</p>
                 <p class="status-msg failure error_messages"></p>
             </div>