chore(demo): fix image path in sharing metadata

public/index.html

@@ -22,7 +22,9 @@
 <meta property="og:site_name" content="Draft.js filters">
 <meta property="og:title" content="Draft.js filters">
 <meta property="og:description" content="Filter Draft.js content to preserve only the formatting you allow. Built for Draftail and Wagtail">
-<meta property="og:image" content="%PUBLIC_URL%/word-toolbars-overload.jpg">
+<meta property="og:image" content="https://thibaudcolas.github.io/draftjs-filters/word-toolbars-overload.jpg">
+<meta property="og:image:width" content="575">
+<meta property="og:image:height" content="462">
 <meta name="twitter:image:alt" content="Screenshot of Microsoft Word with tens of toolbars activated, cluttering the screen with too much functionality">
 <meta name="twitter:label2" value="Paste from Word?">
 <meta name="twitter:data2" value="Yep">