Update donation link

Update donation link from hosted Salsa Labs page (which no longer works)
to page hosted on NumFOCUS site.

Author: martey

templates/donate.html

@@ -17,7 +17,7 @@ <h2 class="title">{% trans %}Support SymPy Development{% endtrans %}</h2>
 
     <section id="get-started">
     <p>{% trans
-    %}<a href="https://numfocus.salsalabs.org/donate-to-sympy/index.html">DONATE
+    %}<a href="https://numfocus.org/donate-to-sympy">DONATE
     NOW</a> {% endtrans %}</p>
   </div>
 </section>