Correct composer option syntax

Author: Olivier Dolbeau

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.