minor #14688 [Routing] clarify deprecation message (xabbuh)

This PR was merged into the 2.7 branch.

Discussion
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[Routing] clarify deprecation message

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

as reported by @javiereguiluz in https://github.com/symfony/symfony/commit/7784b29e5afa4aed419b55e857cf9313627aa408#commitcomment-11229119

Commits
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8a99645 [Routing] clarify deprecation message