update contributions link

Author: mitzimorris

support/index.md

@@ -20,10 +20,10 @@ Stan can now accept tax-deductible contributions through this
 foundation. If you’re interested in contributing to Stan's ongoing
 development, you may contact us directly or just follow the link: 
 
+
 * <p>
-  <a href="https://numfocus.salsalabs.org/donate-to-stan/index.html">Contribute
+  <a href="https://numfocus.org/donate-to-stan">Contribute
   to Stan</a> &nbsp;
-  <span class="note">(Salsa Labs payment processing)</span>
   </p>
 
 It is also possible to donate to Stan via the GitHub sponsors program: