minor #9694 Fixed the project creating command for Symfony 3.4 (javiereguiluz)

This PR was merged into the 3.4 branch.

Discussion
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Fixed the project creating command for Symfony 3.4

A use on Slack warned us about this issue.

Commits
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513c4be Fixed the project creating command for Symfony 3.4

Author: javiereguiluz

setup.rst

@@ -17,7 +17,7 @@ Create your new project by running:
 
 .. code-block:: terminal
 
-    $ composer create-project symfony/website-skeleton my-project
+    $ composer create-project symfony/website-skeleton:^3.4 my-project
 
 This will create a new ``my-project`` directory, download some dependencies into
 it and even generate the basic directories and files you'll need to get started.