update terms of service instructions

static/terms-of-service.html

 <h4>Terms of Service</h4>
 
-<p>This is a placeholder ToS.</p>
+<p>This is the default placeholder ToS.  You should change it to fit the needs of your instance.</p>
 
-<p>Edit <code>"/static/terms-of-service.html"</code> to make it fit the needs of your instance.</p>
+<p>To do so, place a file at <code>"/instance/static/terms-of-service.html"</code> in your
+   Pleroma install containing the real ToS for your instance.</p>
 <br>
-<img src="/static/logo.png"/ style="display: block; margin: auto;">
+<img src="/static/logo.png" style="display: block; margin: auto;">