fix(overmind): fix type related to onMutation dispose return type

Author: christianalfoni

packages/node_modules/overmind/src/derived.ts

@@ -16,7 +16,7 @@ export class Derived {
   private value: any
   private paths: Set<string>
   private updateCount: number = 0
-  private disposeOnMutation: () => {}
+  private disposeOnMutation: () => void
   constructor(private cb: (state: object, parent: object) => void) {
     const boundEvaluate: any = this.evaluate.bind(this)