fix broken link for agenda (#25)

Author: josibake

_posts/2024-08-29-bitdevs.md

@@ -25,7 +25,7 @@ These rules exist so that BitDevs participants can speak freely within the event
 # Agenda
 
 * [v28 release candidates coming soon!](https://github.com/bitcoin/bitcoin/issues/29891)
-  * ["Fire up the guix machine"](github.com/bitcoin-dev-tools/bgt)
+  * ["Fire up the guix machine"](https://github.com/bitcoin-dev-tools/bgt)
   * ["Fixing Lightning Force Closures (with v28)"](https://mblack.io/posts/fixing-lightning-force-closures/)
   * ["AssumeUTXO on mainnet"](https://github.com/bitcoin/bitcoin/pull/28553)
 * Notable PRs