Docs: Fix title of @observable page (#2149)
Docs: Fix title of @observable page (#2149)

The page title for the @observable page in the "Making Things observable"-section was missing an 'r'.
mweststrate/MOBservable,mweststrate/MOBservable,mweststrate/MOBservable,mobxjs/mobx,mobxjs/mobx,mobxjs/mobx,mobxjs/mobx