minor silexphp#1366 Fix installation instruction in readme (jdreesen)

This PR was merged into the 2.0.x-dev branch.

Discussion
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Fix installation instruction in readme

Silex 2 has been released yesterday so `@dev` should be removed from the composer version string.

Commits
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862bd6b Fix installation instruction in readme

README.rst

@@ -27,7 +27,7 @@ The recommended way to install Silex is through `Composer`_:
 
 .. code-block:: bash
 
-    composer require silex/silex "~2.0@dev"
+    composer require silex/silex "~2.0"
 
 Alternatively, you can download the `silex.zip`_ file and extract it.