Fix missing og:image

Author: cheeaun

index.html

@@ -20,7 +20,7 @@
 <meta name="twitter:url" property="og:url" content="https://exploretrees.sg">
 <meta name="twitter:title" property="og:title" content="ExploreTrees.SG">
 <meta name="twitter:description" property="og:description" content="Explore trees in Singapore">
-<meta name="twitter:image" property="og:image" content="https://exploretrees.sg/screenshots/preview.png">
+<meta name="twitter:image" property="og:image" content="https://i.imgur.com/Eh6GK0Q.png">
 <!-- end metacrap -->
 <div id="map"></div>
 <div id="modal"></div>