minor symfony#11596 Update installation.rst (janmyszkier)

This PR was merged into the 3.4 branch.

Discussion
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Update installation.rst

update encore package name, `encore` is not recognized by composer currently

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Commits
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4d82f66 Update installation.rst

Author: javiereguiluz

frontend/encore/installation.rst

@@ -13,7 +13,7 @@ following commands:
 
 .. code-block:: terminal
 
-    $ composer require encore
+    $ composer require symfony/webpack-encore-bundle
     $ yarn install
 
 This will install and enable the `WebpackEncoreBundle`_, add the ``assets/``