docs: Fix grammatical error in types documentation (#1795)
docs: Fix grammatical error in types documentation (#1795)

[skip ci]
mobxjs/mobx-state-tree,mweststrate/mobx-state-tree,mobxjs/mobx-state-tree,mweststrate/mobx-state-tree,mobxjs/mobx-state-tree,mobxjs/mobx-state-tree