servers: update heroku commit pin

We weren't referencing the PR branch correctly.

Signed-off-by: Jacob Howard <jacob.howard@docker.com>

Author: xenoscopic

servers/heroku/server.yaml

@@ -12,7 +12,7 @@ about:
 source:
   project: https://github.com/heroku/heroku-mcp-server
   branch: refs/pull/24/merge
-  commit: 7f956f759cb8b604b98096f701d56d0e22d47c5d
+  commit: f4949381035a04cb862b323ed024762bd2bb12aa
 config:
   secrets:
     - name: heroku.api_key