minor #15244 Fix the filename in the code block (partulaj)

This PR was submitted for the 5.2 branch but it was merged into the 4.4 branch instead.

Discussion
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Fix the filename in the code block

Commits
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b5c7bca Fix the filename in the code block

Author: javiereguiluz

frontend/encore/legacy-applications.rst

@@ -75,7 +75,7 @@ page, add:
 
 .. code-block:: diff
 
-      // webpack.config.js
+      // app.js
 
       // require jQuery normally
       const $ = require('jquery');