minor #15394 Use BaseKernel class alias (sebpacz)

This PR was merged into the 4.4 branch.

Discussion
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Use BaseKernel class alias

This PR fixes a namespace conflict.

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Commits
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b295eeb Use BaseKernel class alias

Author: javiereguiluz

configuration/override_dir_structure.rst

@@ -80,7 +80,7 @@ method::
     // src/Kernel.php
 
     // ...
-    class Kernel extends Kernel
+    class Kernel extends BaseKernel
     {
         // ...