minor #5662 Fixed wrong version of symfony with composer install (Nek-)

This PR was merged into the 2.7 branch.

Discussion
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Fixed wrong version of symfony with composer install

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | >2.7
| Fixed tickets | #5627 (comment)

Commits
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a775e38 Fixed wrong version of symfony with composer install

Author: wouterj

book/installation.rst

@@ -95,13 +95,12 @@ optional second argument of the ``new`` command:
 .. code-block:: bash
 
     # use the most recent version in any Symfony branch
-    $ symfony new my_project_name 2.3
-    $ symfony new my_project_name 2.5
     $ symfony new my_project_name 2.6
+    $ symfony new my_project_name 2.7
 
     # use a specific Symfony version
-    $ symfony new my_project_name 2.3.26
     $ symfony new my_project_name 2.6.5
+    $ symfony new my_project_name 2.7.3
 
     # use a beta or RC version (useful for testing new Symfony versions)
     $ symfony new my_project 2.7.0-BETA1
@@ -151,7 +150,7 @@ version as the second argument of the ``create-project`` command:
 
 .. code-block:: bash
 
-    $ composer create-project symfony/framework-standard-edition my_project_name "2.3.*"
+    $ composer create-project symfony/framework-standard-edition my_project_name "2.7.*"
 
 .. tip::