minor symfony#6481 Remove reference to Symfony2 in request-flow.png (Daniel Cotton)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes symfony#6481).

Discussion
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Remove reference to Symfony2 in request-flow.png

As per discussion in symfony#6480, update the routing flow graphic to remove the reference to Symfony2.

Commits
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ae8a340 Remove reference to Symfony2 in request-flow.png

Author: wouterj