add init-db call to flask tutorial

otherwise the user is confronted with an unpleasant error
message when accessing the dev server the first time

Author: senarclens

examples/tutorial/README.rst

@@ -49,12 +49,14 @@ Run
 
     export FLASK_APP=flaskr
     export FLASK_ENV=development
+    flask init-db
     flask run
 
 Or on Windows cmd::
 
     set FLASK_APP=flaskr
     set FLASK_ENV=development
+    flask init-db
     flask run
 
 Open http://127.0.0.1:5000 in a browser.