minor symfony#8286 Enhanced custom plugin configuration (martijngastkemper)

This PR was merged into the 3.3 branch.

Discussion
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Enhanced custom plugin configuration

The example doesn't include a declaration of variable ``webpack``.

Commits
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df8f028 Add require('webpack') to prevent undefined var

Author: xabbuh

frontend/encore/custom-loaders-plugins.rst

@@ -51,6 +51,8 @@ to use the `IgnorePlugin`_ (see `moment/moment#2373`_):
 .. code-block:: diff
 
     // webpack.config.js
+    + var webpack = require('webpack');
+    
     Encore
         // ...