minor #10923 Fix encore advanced config watchOptions example (eXtreme)

This PR was merged into the 3.4 branch.

Discussion
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Fix encore advanced config watchOptions example

I've been struggling with this for some time and couldn't get `watchOptions` to work. After a long investigation I added `devServer` to the config and it started working.

So I guess this is the correct example of adding watchOptions to the config.

Commits
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06f5c23 Fix encore advanced config watchOptions example

Author: javiereguiluz

frontend/encore/advanced-config.rst

@@ -18,7 +18,7 @@ modify the config after fetching it from Encore:
 
     // fetch the config, then modify it!
     var config = Encore.getWebpackConfig();
-    config.watchOptions = { poll: true, ignored: /node_modules/ };
+    config.devServer.watchOptions = { poll: true, ignored: /node_modules/ };
 
     // other examples: add an alias or extension
     // config.resolve.alias.local = path.resolve(__dirname, './resources/src');