slots migration guide, mention change to function more prominently (vuejs#452)

* slots migration guide, mention change to function more prominently

see vuejs/core#2045

* Update src/guide/migration/slots-unification.md

Co-authored-by: Natalia Tepluhina <[email protected]>

Co-authored-by: Natalia Tepluhina <[email protected]>

Author: Haroenv

src/guide/migration/slots-unification.md

@@ -54,11 +54,12 @@ And when you need to reference scoped slots programmatically, they are now unifi
 this.$scopedSlots.header
 
 // 3.x Syntax
-this.$slots.header
+this.$slots.header()
 ```
 
 ## Migration Strategy
 
 A majority of the change has already been shipped in 2.6. As a result, the migration can happen in one step:
 
 1. Replace all `this.$scopedSlots` occurrences with `this.$slots` in 3.x.
+2. Replace all occurences of `this.$slots.mySlot` with `this.$slots.mySlot()`