Update protocols/gossipsub/CHANGELOG.md

Co-authored-by: Darius Clark <dariusc93@users.noreply.github.com>

Author: jxs

protocols/gossipsub/CHANGELOG.md

@@ -1,6 +1,6 @@
 ## 0.49.0
 - Feature gate metrics related code. This changes some `Behaviour` constructor methods.
-  See [PR XXXX](https://github.com/libp2p/rust-libp2p/pull/XXXX)
+  See [PR 6020](https://github.com/libp2p/rust-libp2p/pull/6020)
 - Send IDONTWANT before Publishing a new message.
   See [PR 6017](https://github.com/libp2p/rust-libp2p/pull/6017)