Update derive.md

fix typo

Author: tuliglowicz

docs/concept/derive.md

@@ -17,7 +17,7 @@ head:
 # Derive
 Just like _state_ and _decorate_, derive allows you to customize `Context` based on existing `Context` as `Context.store`.
 
-This is useful when you to need to create a value based on an existing value reactively.
+This is useful when you need to create a value based on an existing value reactively.
 
 ### Example
 ```typescript