Add missing parameter in mocking zustand testing guide (pmndrs#2142)

It seems that the parameter was missing here, resulting in returning createUncurried instead of createUncurried(stateCreator)

docs/guides/testing.md

@@ -87,7 +87,7 @@ const createUncurried = <T>(stateCreator: zustand.StateCreator<T>) => {
 }
 
 // when creating a store, we get its initial state, create a reset function and add it in the set
-export const create = (<T>() => {
+export const create = (<T>(stateCreator: zustand.StateCreator<T>) => {
   console.log('zustand create mock')
 
   // to support curried version of create