Update README.md

Fix significant typo in "test your application without security" hint.  The old guidance led to "An Authentication object was not found in the SecurityContext".  More info is also available in Jacob Fund's October 28, 2014 thread "Problem authenticating to host", https://class.coursera.org/androidcapstone-001/forum/thread?thread_id=342#post-1384.
Merge pull request #41 from buzz3791/patch-1

Update README.md