theme: Update open graph image

Author: Will Binns

_theme/layout.html

@@ -11,7 +11,7 @@
     <meta property="og:description" content="This site aims to provide the
     docs you need to understand Bitcoin and start building Bitcoin-based
     applications." />
-    <meta property="og:image" content="https://bitcoindocumentation.org/_static/bitcoindocs.png" />
+    <meta property="og:image" content="https://developer.bitcoin.org/_static/bitcoindocs.png" />
     <meta property="og:type" content="website" />
     <link href="https://fonts.googleapis.com/css?family=Titillium+Web:400,700&display=swap" rel="stylesheet">
     <!-- Global site tag (gtag.js) - Google Analytics -->
@@ -130,4 +130,4 @@
         </div>
       </div>
     </div>
-{%- endblock %}
+{%- endblock %}