minor symfony#4999 Fixed semantic error (beni0888)

This PR was merged into the 2.3 branch.

Discussion
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Fixed semantic error

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets |

Commits
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dd47e1d Fixed semantic error

Author: weaverryan

cookbook/bundles/extension.rst

@@ -70,7 +70,7 @@ should also override
 :method:`Extension::getAlias() <Symfony\\Component\\DependencyInjection\\Extension\\Extension::getAlias>`
 to return the correct DI alias. The DI alias is the name used to refer to the
 bundle in the container (e.g. in the ``app/config/config.yml`` file). By
-default, this is done by removing the ``Extension`` prefix and converting the
+default, this is done by removing the ``Extension`` suffix and converting the
 class name to underscores (e.g. ``AcmeHelloExtension``'s DI alias is
 ``acme_hello``).