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():
@@ -34,6 +38,10 @@ So here is a full example::
                 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