fix example screenshot path

Author: acao

dev/index.html

@@ -18,8 +18,8 @@
     <meta name="twitter:card" content="summary" />
     <meta name="og:title" content="codemirror-json-schema example" />
     <meta name="og:type" content="website" />
-    <meta name="og:image" content="./assets/example.png" />
-    <meta name="twitter:image" content="./assets/example.png" />
+    <meta name="og:image" content="/example.png" />
+    <meta name="twitter:image" content="/example.png" />
     <style>
       body {
         background-color: #282c34;