Edit docs/configuring-playbook-bridge-hookshot.md: add optional label to GitHub private key instruction

Signed-off-by: Suguru Hirahara <acioustick@noreply.codeberg.org>

docs/configuring-playbook-bridge-hookshot.md

@@ -10,7 +10,7 @@ See the project's [documentation](https://matrix-org.github.io/matrix-hookshot/l
 
 ## Prerequisites
 
-### Download GitHub app private key
+### Download GitHub app private key (optional)
 
 If you're setting up the GitHub bridge, you need to create your GitHub app, and generate a private key file of it.