chore: correct upgrade doc title

Author: achingbrain

doc/migrations/v2.0.0-v3.0.0.md

@@ -1,5 +1,5 @@
 <!--Specify versions for migration below-->
-# Migrating to libp2p@2.0 <!-- omit in toc -->
+# Migrating to libp2p@3.0 <!-- omit in toc -->
 
 A migration guide for refactoring your application code from libp2p `v2.0.0` to `v3.0.0`.