Fix 404 in link to API documentation from readme
Fix 404 in link to API documentation from readme

https://mobxjs.github.io/ 404s and does not actually link to the API documentation, so here's a fix!
mweststrate/MOBservable,andykog/mobservable,mobxjs/mobx,andykog/mobservable,andykog/mobx,mobxjs/mobx,mobxjs/mobx,andykog/mobx,mweststrate/MOBservable,mweststrate/MOBservable,mobxjs/mobx