Use print function in quickstart (#2204)

Example in URL Building section uses `print` statement
instead of `print` function, which causes syntax error
when example is run on Python 3.

docs/quickstart.rst

@@ -264,10 +264,10 @@ some examples::
     ... def profile(username): pass
     ...
     >>> with app.test_request_context():
-    ...  print url_for('index')
-    ...  print url_for('login')
-    ...  print url_for('login', next='/')
-    ...  print url_for('profile', username='John Doe')
+    ...  print(url_for('index'))
+    ...  print(url_for('login'))
+    ...  print(url_for('login', next='/'))
+    ...  print(url_for('profile', username='John Doe'))
     ...
     /
     /login