Fix command for installing with Go toolchain (versioned)

Thanks pointing out the issue @JKKGBE !

Author: Prometheus2677

cmd/migrate/README.md

@@ -41,7 +41,7 @@ $ apt-get install -y migrate
 $ go get -u -d github.com/golang-migrate/migrate/cmd/migrate
 $ cd $GOPATH/src/github.com/golang-migrate/migrate/cmd/migrate
 $ git checkout $TAG  # e.g. v4.1.0
-$ go build -tags 'postgres' -ldflags="-X main.Version=$(git describe --tags)" -o $GOPATH/bin/migrate github.com/golang-migrate/migrate/cmd/migrate
+$ go build -tags 'postgres' -ldflags="-X main.Version=$(git describe --tags)" -o $GOPATH/bin/migrate $GOPATH/src/github.com/golang-migrate/migrate/cmd/migrate
 ```
 
 #### Unversioned