Update onBeforeUnmount type (vuejs#1580)

There is a typo. The method name in Type section is onBeforeUnmounted (the correct name is onBeforeUnmount).

Author: florentinap

src/api/composition-api-lifecycle.md

@@ -167,7 +167,7 @@ Registers a hook to be called right before a component instance is to be unmount
 - **Type**
 
   ```ts
-  function onBeforeUnmounted(callback: () => void): void
+  function onBeforeUnmount(callback: () => void): void
   ```
 
 - **Details**