minor #5805 Mentioned the BETA and RC support for the Symfony Installer (javiereguiluz)

This PR was merged into the 2.3 branch.

Discussion
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Mentioned the BETA and RC support for the Symfony Installer

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | all
| Fixed tickets | #5734

Commits
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02daba0 Mentioned the BETA and RC support for the Symfony Installer

Author: weaverryan

book/installation.rst

@@ -98,17 +98,17 @@ optional second argument of the ``new`` command:
     $ symfony new my_project_name 2.3.26
     $ symfony new my_project_name 2.6.5
 
-If you want your project to be based on the latest :ref:`Symfony LTS version <releases-lts>`,
-pass ``lts`` as the second argument of the ``new`` command:
+    # use a beta or RC version (useful for testing new Symfony versions)
+    $ symfony new my_project 2.7.0-BETA1
+    $ symfony new my_project 2.7.0-RC1
+
+The installer also supports a special version called ``lts`` which installs the
+most recent :ref:`Symfony LTS version <releases-lts>` available:
 
 .. code-block:: bash
 
-    # Linux, Mac OS X
     $ symfony new my_project_name lts
 
-    # Windows
-    c:\projects\> php symfony new my_project_name lts
-
 Read the :doc:`Symfony Release process </contributing/community/releases>`
 to better understand why there are several Symfony versions and which one
 to use for your projects.