Update nodejs_web_app.md

Updated to documentation to use the centos6 image instead of the unofficial tutum image of centos6.4

docs/sources/examples/nodejs_web_app.md

@@ -66,10 +66,10 @@ requires to build (this example uses Docker 0.3.4):
 
 Next, define the parent image you want to use to build your own image on
 top of. Here, we'll use
-[CentOS](https://registry.hub.docker.com/_/centos/) (tag: `6.4`)
+[CentOS](https://registry.hub.docker.com/_/centos/) (tag: `centos6`)
 available on the [Docker Hub](https://hub.docker.com/):
 
-    FROM    tutum/centos-6.4
+    FROM    centos:centos6
 
 Since we're building a Node.js app, you'll have to install Node.js as
 well as npm on your CentOS image. Node.js is required to run your app
@@ -109,7 +109,7 @@ defines your runtime, i.e. `node`, and the path to our app, i.e. `src/index.js`
 Your `Dockerfile` should now look like this:
 
     # DOCKER-VERSION 0.3.4
-    FROM    tutum/centos-6.4
+    FROM    centos:centos6
 
     # Enable EPEL for Node.js
     RUN     rpm -Uvh http://download.fedoraproject.org/pub/epel/6/i386/epel-release-6-8.noarch.rpm
@@ -137,9 +137,9 @@ Your image will now be listed by Docker:
     $ sudo docker images
 
     # Example
-    REPOSITORY                            TAG        ID              CREATED
-    tutum                                 centos-6.4 539c0211cd76    8 weeks ago
-    <your username>/centos-node-hello     latest     d64d3505b0d2    2 hours ago
+    REPOSITORY                          TAG        ID              CREATED
+    centos                              centos6    539c0211cd76    8 weeks ago
+    <your username>/centos-node-hello   latest     d64d3505b0d2    2 hours ago
 
 ## Run the image