minor #5680 fix grammar mistake (greg0ire)

This PR was merged into the 2.3 branch.

Discussion
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fix grammar mistake

Commits
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91f9b8a fix grammar mistake

components/dependency_injection/synthetic_services.rst

@@ -27,7 +27,7 @@ service is injected into the container from within the ``Kernel`` class::
 The ``kernel`` service is called a synthetic service. This service has to
 be configured in the container, so the container knows the service does
 exist during compilation (otherwise, services depending on this ``kernel``
-service will get a "service does not exists" error).
+service will get a "service does not exist" error).
 
 In order to do so, you have to use
 :method:`Definition::setSynthetic() <Symfony\\Component\\DependencyInjection\\Definition::setSynthetic>`::