Build instructions in README.md are incorrect

[brianredbeard]

methinks that based on #2 showing that this endpoint has been having issues and the details therein that the information in README.md about installation is incorrect:

https://github.com/square/gssh/blob/3f6030867087252fda48ca47eee299bd7ea1082a/README.md#L34-L38

e.g.:

$ go get -u -v github.com/square/gssh
github.com/square/gssh (download)
github.com/square/erg (download)
github.com/square/grange (download)
github.com/deckarep/golang-set (download)
github.com/orcaman/concurrent-map (download)
unrecognized import path "vbom.ml/util/sortorder": reading https://vbom.ml/util/sortorder?go-get=1: 503 Service Unavailable
$ echo $?
1