Fix instructions for installing goreman.

Signed-off-by: James Blair <mail@jamesblair.net>

Author: jmhbnz

content/en/docs/v3.4/dev-guide/local_cluster.md

@@ -52,7 +52,7 @@ A `Procfile` at the base of the etcd git repository is provided to easily config
 1. Install `goreman` to control Procfile-based applications:
 
     ```
-    $ go get github.com/mattn/goreman
+    $ go install github.com/mattn/goreman@latest
     ```
 
 2. Start a cluster with `goreman` using etcd's stock Procfile:

content/en/docs/v3.5/dev-guide/local_cluster.md

@@ -52,7 +52,7 @@ A `Procfile` at the base of the etcd git repository is provided to easily config
 1. Install `goreman` to control Procfile-based applications:
 
     ```
-    $ go get github.com/mattn/goreman
+    $ go install github.com/mattn/goreman@latest
     ```
 
 2. Start a cluster with `goreman` using etcd's stock Procfile:

content/en/docs/v3.6/dev-guide/local_cluster.md

@@ -52,7 +52,7 @@ A `Procfile` at the base of the etcd git repository is provided to easily config
 1. Install `goreman` to control Procfile-based applications:
 
     ```
-    $ go get github.com/mattn/goreman
+    $ go install github.com/mattn/goreman@latest
     ```
 
 2. Start a cluster with `goreman` using etcd's stock Procfile: