fix link to v2 docs (FriendlyCaptcha#279)

Author: merlinfuchs

docs/_sidebar.md

@@ -15,4 +15,4 @@
     * [ 📘 Friendly Captcha website](https://friendlycaptcha.com)
     * [ 🗂️ Github repository](https://github.com/friendlycaptcha/friendly-challenge)
     * [ 📦 NPM package](https://www.npmjs.com/package/friendly-challenge)
-    * [ ⚡️ V2 Documentation](https://developer.friendlycaptcha.com/docs/)
+    * [ ⚡️ V2 Documentation](https://developer.friendlycaptcha.com/docs/v2)