minor docs tweak: Source => Open source

templates/base.html

@@ -56,7 +56,7 @@ to <a href="https://joinmastodon.org/">Mastodon</a> and the
 <div id="footer">
   <a href="/docs">Docs</a>
   | <a href="https://github.com/snarfed/bridgy-fed/issues">Bug?</a>
-  | <a href="https://github.com/snarfed/bridgy-fed">Source</a>
+  | <a href="https://github.com/snarfed/bridgy-fed">Open source</a>
   | <a href="https://indieweb.org/">#IndieWeb</a>
   | <a href="https://brid.gy/">Bridgy</a>
 </div>