lightningd: fix obsolete comment.

We unified this timeout with the 60 second startup timeout.

Reported-by: @cdecker
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

lightningd/plugin_control.c

@@ -53,7 +53,7 @@ struct command_result *plugin_cmd_all_complete(struct plugins *plugins,
 
 /**
  * Called when trying to start a plugin through RPC, it starts the plugin and
- * will give a result 20 seconds later at the most.
+ * will give a result 60 seconds later at the most (once init completes).
  */
 static struct command_result *
 plugin_dynamic_start(struct command *cmd, const char *plugin_path)