docs(user-event): add missing return type (#1128)

Author: ph-fritsche

docs/user-event/api-convenience.mdx

@@ -70,7 +70,7 @@ pointer({target: element.ownerDocument.body})
 ### tab()
 
 ```ts
-tab(options: {shift?: boolean})
+tab(options: {shift?: boolean}): Promise<void>
 ```
 
 ```js