Fix Message Flashing doc, from SwashBuckla #pocoo.

Provide a full example as promised in the doc.

docs/patterns/flashing.rst

@@ -16,7 +16,11 @@ Simple Flashing
 
 So here is a full example::
 
-    from flask import flash, redirect, url_for, render_template
+    from flask import Flask, flash, redirect, render_template, \
+         request, url_for
+
+    app = Flask(__name__)
+    app.secret_key = 'some_secret'
 
     @app.route('/')
     def index():
@@ -27,13 +31,17 @@ So here is a full example::
         error = None
         if request.method == 'POST':
             if request.form['username'] != 'admin' or \
-               request.form['password'] != 'secret':
+                    request.form['password'] != 'secret':
                 error = 'Invalid credentials'
             else:
                 flash('You were successfully logged in')
                 return redirect(url_for('index'))
         return render_template('login.html', error=error)
 
+    if __name__ == "__main__":
+        app.run()
+
+
 And here the ``layout.html`` template which does the magic:
 
 .. sourcecode:: html+jinja