minor #13314 [Form] tweaked a deprecation message (fabpot)
minor #13314 [Form] tweaked a deprecation message (fabpot)

This PR was merged into the 2.7 branch.

Discussion
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[Form] tweaked a deprecation message

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

While upgrading an app to 2.7, I found this message unclear. To be honest, and like many developers, I did not read the entire message and I missed the "cast to a string" part.

Commits
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a396b41 [Form] tweaked a deprecation message

symfony/Form