Update introduction.rst

Found that use Symfony\Component\HttpKernel\EventListener\RouterListener; is missing
 in full working example.
bug #3550 Update introduction.rst (taavit)

This PR was submitted for the 2.4 branch but it was merged into the 2.3 branch instead (closes #3550).

Discussion
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Update introduction.rst

Found that use Symfony\Component\HttpKernel\EventListener\RouterListener; is missing
 in full working example.

Commits
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89f23be Update introduction.rst