Update mercure.rst

Co-authored-by: Wouter J <[email protected]>

Author: dunglas

mercure.rst

@@ -73,7 +73,7 @@ On Linux and Mac, run the following command to start it:
 .. rst-class:: command-linux
 .. code-block:: terminal
 
-    SERVER_NAME=:3000 MERCURE_PUBLISHER_JWT_KEY="!ChangeMe!" MERCURE_SUBSCRIBER_JWT_KEY="!ChangeMe!" ./mercure run -config Caddyfile.dev
+    $ SERVER_NAME=:3000 MERCURE_PUBLISHER_JWT_KEY="!ChangeMe!" MERCURE_SUBSCRIBER_JWT_KEY="!ChangeMe!" ./mercure run -config Caddyfile.dev
 
 On Windows run: