bug #8638 Correct composer option syntax (odolbeau)

This PR was merged into the master branch.

Discussion
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Correct composer option syntax

Commits
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37acb46 Correct composer option syntax

Author: wouterj

setup/unstable_versions.rst

@@ -15,10 +15,10 @@ execute the following command:
 .. code-block:: terminal
 
     # Download the latest beta version
-    $ composer create-project symfony/skeleton my_project "4.0.*" -s=beta
+    $ composer create-project symfony/skeleton my_project "4.0.*" -s beta
     
     # Download the absolute latest commit
-    $ composer create-project symfony/skeleton my_project "4.0.*" -s=dev
+    $ composer create-project symfony/skeleton my_project "4.0.*" -s dev
 
 Once the command finishes its execution, you'll have a new Symfony project created
 in the ``my_project/`` directory.