fix(overmind): fix typing issue with reaction

Author: christianalfoni

packages/node_modules/overmind/src/Overmind.ts

@@ -8,7 +8,7 @@ import { proxifyEffects } from './proxyfyEffects'
 import { rehydrate } from './rehydrate'
 import { IConfiguration } from './types'
 import * as utils from './utils'
-import { IReaction } from './types'
+import { IReaction, IContext } from './types'
 
 const hotReloadingCache = {}
 
@@ -807,7 +807,7 @@ export class Overmind<ThisConfig extends IConfiguration>
   getMutationTree(): proxyStateTree.IMutationTree<any> {
     return this.proxyStateTreeInstance.getMutationTree()
   }
-  reaction: IReaction<ThisConfig> = (
+  reaction: IReaction<IContext<ThisConfig>> = (
     stateCallback,
     updateCallback,
     options = {}