Update script installation path in README

[sethbergman]

From the README:

Put the (self-contained) sbt script somewhere on your path, for instance:

curl -Ls https://git.io/sbt > ~/bin/sbt && chmod 0755 ~/bin/sbt

Terminal output on ubuntu:18.04:

$ curl -Ls https://git.io/sbt > ~/bin/sbt && chmod 0755 ~/bin/sbt
bash: /root/bin/sbt: No such file or directory

[dwijnand]

#285 (comment)