vignette links in REAMDME.md are broken

Thanks for making (and maintaining) this awesome package. I wanted to let you know that the vignette links in the readme are broken (i.e. the link to  2. US State Choropleths is [broken](https://cran.r-project.org/web/packages/choroplethr/vignettes/b-state-choropleth.html)