Update index.md with spelling fix (#1747 by @Yamo93)
Update index.md with spelling fix (#1747 by @Yamo93)

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