minor symfony#13158 fixing typo (abdounikarim)

This PR was submitted for the 5.0 branch but it was merged into the 3.4 branch instead (closes symfony#13158).

Discussion
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fixing typo

Commits
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670dcfe fixing typo

Author: javiereguiluz

frontend/encore/simple-example.rst

@@ -332,7 +332,7 @@ If you want to only compile a CSS file, that's possible via ``addStyleEntry()``:
 
 .. code-block:: javascript
 
-    // webpack/config.js
+    // webpack.config.js
     Encore
         // ...