Fixed link to "What is an integration?"

The current link is pointing to http://devdocs.magento.com/guides/v1.0/config-guide/integration/what-is-integration.html but the file can be found at http://devdocs.magento.com/guides/v1.0/config-guide/integration/integration-what-is.html.

I hope I edited the markdown file in the correct way.