Clarify Amazon EC2 installation type

Make clear this documentation is about installing docker on EC2 with the
help of vagrant, which doesn't make it easy to start multiple instances,
instead of plain vanilla EC2 installation.

docs/sources/installation/amazon.rst

@@ -1,9 +1,9 @@
-:title: Installation on Amazon EC2 
+:title: Installation on Amazon EC2
 :description: Docker installation on Amazon EC2 with a single vagrant command. Vagrant 1.1 or higher is required.
 :keywords: amazon ec2, virtualization, cloud, docker, documentation, installation
 
-Amazon EC2
-==========
+Using Vagrant (Amazon EC2)
+==========================
 
   Please note this is a community contributed installation path. The only 'official' installation is using the
   :ref:`ubuntu_linux` installation path. This version may sometimes be out of date.