fix: social cards (#65)

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## Summary by CodeRabbit

* **Style**
  * Updated the social preview image used for sharing the site.

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Author: th0rOdinson

sites/optimism/docusaurus.config.ts

@@ -40,7 +40,7 @@ const localConfig: Config = {
   ],
 
   themeConfig: {
-    image: "img/op-handbook-social.svg",
+    image: "img/op-social-card.png",
     navbar: {
       title: "",
       logo: {