Incorrect install instructions on the README.md

[nickschurch]

The current install instructions don't work:

library(devtools)
install_github("FrancescaPianosi/SAFE-R")

produces the error:

Loading required package: usethis
Error: Failed to install 'unknown package' from GitHub:
  HTTP error 404.
  Not Found

  Did you spell the repo owner (`FrancescaPianosi`) and repo name (`SAFE-R`) correctly?
  - If spelling is correct, check that you have the required permissions to access the repo.

The correct install instructions are now:

library(devtools)
install_github("SAFEtoolbox/SAFE-R")

[JKupzig]

For me, install_github("https://github.com/SAFEtoolbox/SAFE-R") worked as well.