minor symfony#10159 Update name of the webpack encore package (razbounak)

This PR was merged into the 3.4 branch.

Discussion
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Update name of the webpack encore package

webpack-encore is not used in composer

Commits
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1e9863c Update name of the webpack encore package

Author: javiereguiluz

frontend/encore/installation.rst

@@ -10,7 +10,7 @@ run:
 
 .. code-block:: terminal
 
-    $ composer require webpack-encore
+    $ composer require symfony/webpack-encore-pack
     $ yarn install
 
 This will create a ``webpack.config.js`` file, add the ``assets/`` directory, and