[doc] startup events are executed for each threads

docs/event.md

@@ -1,6 +1,6 @@
 # Event
 
-`Prologue` supports both `startup` and `shutdown` events. `startup` events will be only executed once before the main loop. In contrast, `shutdown` events will be executed once after the main loop.
+`Prologue` supports both `startup` and `shutdown` events. `startup` events will be only executed once for each threads. In contrast, `shutdown` events will be executed once after the main loop.
 
 Let's first look at the structure of `Event`, you can see that `Event` supports both synchronous and asynchronous closure function pointers.