minor #12658 Update readme (noniagriconomie)

This PR was submitted for the master branch but it was merged into the 3.4 branch instead (closes #12658).

Discussion
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Update readme

I think `by` means like if SymfonyCloud is a CI or something like that
no? (feel free to close)

Commits
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c009c1b Update readme

Author: javiereguiluz

README.markdown

@@ -18,7 +18,7 @@ Symfony documentation, please read
 SymfonyCloud
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-Pull requests are automatically built by [SymfonyCloud](https://symfony.com/cloud).
+Pull requests are automatically built on [SymfonyCloud](https://symfony.com/cloud).
 
 Docker
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