Documentation: path "~/go" to "$GOPATH"

In "Install from source" section supposed that Grafana`s sources installed to `~/go` , but it is not always true. 
In other hand, `go get ..` command would install project to `$GOPATH`

https://golang.org/cmd/go/#hdr-Download_and_install_packages_and_dependencies

README.md

@@ -33,7 +33,7 @@ the latest master builds [here](https://grafana.com/grafana/download)
 ### Building the backend
 ```bash
 go get github.com/grafana/grafana
-cd ~/go/src/github.com/grafana/grafana
+cd $GOPATH/src/github.com/grafana/grafana
 go run build.go setup
 go run build.go build
 ```