Improve button wording on speaker management page

"Cancel" implied that the button would revert changes you'd made to add
speakers.

pinaxcon/templates/symposion/proposals/proposal_speaker_manage.html

@@ -23,7 +23,7 @@
         <fieldset>
           {% include "_form_snippet.html" with form=add_speaker_form %}
           <input type="submit" value="Add speaker" class="btn btn-primary"/>
-          <a href="{% url "proposal_edit" proposal.pk %}" class="btn btn-light">{% trans 'Cancel' %}</a>
+          <a href="{% url "proposal_edit" proposal.pk %}" class="btn btn-light">{% trans 'Continue editing proposal' %}</a>
         </fieldset>
     </form>
 {% endblock %}