fix: correct the default value in the agentCardPath comment (#130)

# Description
The default value of `agentCardPath` does not have a preceding slash.

https://github.com/a2aproject/a2a-js/blob/c055b769d96cd197dfb89748c4708277de66684c/src/constants.ts#L8

Author: hujiulong

src/server/express/a2a_express_app.ts

@@ -20,7 +20,7 @@ export class A2AExpressApp {
      * @param app Optional existing Express app.
      * @param baseUrl The base URL for A2A endpoints (e.g., "/a2a/api").
      * @param middlewares Optional array of Express middlewares to apply to the A2A routes.
-     * @param agentCardPath Optional custom path for the agent card endpoint (defaults to /.well-known/agent-card.json).
+     * @param agentCardPath Optional custom path for the agent card endpoint (defaults to .well-known/agent-card.json).
      * @returns The Express app with A2A routes.
      */
     public setupRoutes(