docs(guide/migration/watch.md): fix typo (#743)

Author: AMTazi

src/guide/migration/watch.md

@@ -8,7 +8,7 @@ badges:
 
 ## Overview
 
-- **BREAKING**: When watching an array, the callback will only trigger when the array is replaced. If you need to to trigger on mutation, the `deep` option must be specified.
+- **BREAKING**: When watching an array, the callback will only trigger when the array is replaced. If you need to trigger on mutation, the `deep` option must be specified.
 
 ## 3.x Syntax