Update nextjs.mdx to make Project Token hyperlink work (#1789)

Author: abraibrai

pages/docs/quickstart/install-mixpanel/nextjs.mdx

@@ -12,7 +12,7 @@ import { dataItems } from "/utils/constants";
 
    a. Create a `.env.local` file in the root of your Next.js project (note: for production use `.env.production`).
 
-   b. Then, add your Mixpanel [project token] (https://docs.mixpanel.com/docs/orgs-and-projects/managing-projects#find-your-project-tokens).
+   b. Then, add your Mixpanel [project token](/docs/orgs-and-projects/managing-projects#find-your-project-tokens).
 
    ```javascript
    // Paste in your token