Update conventions.rst

The ``trigger_error``call seems to be written in one line:  https://github.com/symfony/symfony/pull/12671/files
minor #4542 Update conventions.rst (csuarez)

This PR was submitted for the 2.6 branch but it was merged into the 2.3 branch instead (closes #4542).

Discussion
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Update conventions.rst

The ``trigger_error`` call seems to be written in one line:  https://github.com/symfony/symfony/pull/12671/files

Commits
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3d75f6a Update conventions.rst