fix(docs): bracket $HOME properly in "deisctl config" example

Author: mboersma

docs/installing_deis/install-platform.rst

@@ -60,7 +60,7 @@ hosts during ``deis run``:
 
 .. note::
 
-    For Vagrant clusters: ``deisctl config platform set sshPrivateKey=$(HOME)/.vagrant.d/insecure_private_key``
+    For Vagrant clusters: ``deisctl config platform set sshPrivateKey=${HOME}/.vagrant.d/insecure_private_key``
 
 We'll also need to tell the controller which domain name we are deploying applications under: