Fix typo in gatsby-remark-code-repls sample gatsby-config.json in R…

…EADME (gatsbyjs#10361)

Sample `gatsby-config.json` in `gatsby-remark-code-repls` had incorrect JSON in README due to an extra closing parenthesis.

packages/gatsby-remark-code-repls/README.md

@@ -155,7 +155,7 @@ specified examples directory. (This will avoid broken links at runtime.)
     // Optional path to a custom redirect template.
     // The redirect page is only shown briefly,
     // But you can use this setting to override its CSS styling.
-    redirectTemplate: `${__dirname}/src/redirect-template.js`),
+    redirectTemplate: `${__dirname}/src/redirect-template.js`,
 
     // Optional link target.
     // Note that if a target is specified, "noreferrer" will also be added.