Database improperly closed in example code

os.unlink(flaskr.DATABASE) causes the actual application database to be
purged; whereas, I reckon, one wants the _test_ database to be removed.
So every time I ran it, the test passed, but ended up crashing the live
app for want of a valid database.

I avoided using the sample code in examples/flaskr thus far, as I chose
to type out code from the turorial docs. The actual example code looks
good - at least to my beginner's eye.

docs/testing.rst

@@ -102,7 +102,7 @@ test method to our class, like this::
 
         def tearDown(self):
             os.close(self.db_fd)
-            os.unlink(flaskr.DATABASE)
+            os.unlink(flaskr.app.config['DATABASE'])
 
         def test_empty_db(self):
             rv = self.app.get('/')