Fix typos in middleware.md and point to updated CodeSandbox example (#1154)
Fix typos in middleware.md and point to updated CodeSandbox example (#1154)


mobxjs/mobx-state-tree,mobxjs/mobx-state-tree,mweststrate/mobx-state-tree,mweststrate/mobx-state-tree,mobxjs/mobx-state-tree,mobxjs/mobx-state-tree