Fix "undefined app" error in introduction.rst

doc/introduction.rst

@@ -16,11 +16,11 @@ The first step is to initialize an empty admin interface for your Flask app::
     from flask import Flask
     from flask_admin import Admin
 
+    app = Flask(__name__)
+
     # set optional bootswatch theme
     app.config['FLASK_ADMIN_SWATCH'] = 'cerulean'
 
-    app = Flask(__name__)
-
     admin = Admin(app, name='microblog', template_mode='bootstrap3')
     # Add administrative views here