Update default cohort language (#12695)

lms/templates/instructor/instructor_dashboard_2/send_email.html

@@ -38,7 +38,7 @@ from openedx.core.djangolib.markup import HTML
                         %if cohort.name == section_data['default_cohort_name']:
                         <br/>
                         <div class="subheading">
-                            ${_("(Students without cohort assignment)")}
+                            ${_("(Learners not explicitly assigned to a cohort)")}
                         </div>
                         %endif
                     </label>