minor #4993 Bumped symfony version number to 2.6 in flat php composer…

….json example (TSchuermans)

This PR was merged into the 2.6 branch.

Discussion
----------

Bumped symfony version number to 2.6 in flat php composer.json example

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.6.*
| Fixed tickets | /

Commits
-------

b8275b3 Bumped symfony version number to 2.6 in flat php composer.json example

book/from_flat_php_to_symfony2.rst

@@ -435,7 +435,7 @@ content:
 
     {
         "require": {
-            "symfony/symfony": "2.5.*"
+            "symfony/symfony": "2.6.*"
         },
         "autoload": {
             "files": ["model.php","controllers.php"]