Update nodejs_web_app.md

The image centos:centos6.4 doesn't exist. Instead you have to choose between the official centos6 or centos7. Both images does not work together with de yum npm install. When choosing an centos6.5 from another distributor (tutum) it works.
Update nodejs_web_app.md

Updated to documentation to use the centos6 image instead of the unofficial tutum image of centos6.4
Merge pull request #7278 from Toebez/patch-1

Update nodejs_web_app.md