Correct SMART Health IT sandbox link

[amcginn]

The tutorial page [1] has a link under the launch-smart-sandbox.html description that points to a URL [2] that errors when loaded. When loading the main SMART Health IT site, the given sandbox links redirect to a different URL [3].

[1] https://github.com/cerner/smart-on-fhir-tutorial/blob/0d011e9d5542ccd595f5f1d71d550a1e3fc8638c/source/index.html.md
[2] https://sandbox.smarthealthit.org/
[3] http://docs.smarthealthit.org/