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

Author: wouterj

contributing/code/conventions.rst

@@ -103,7 +103,4 @@ A PHP ``E_USER_DEPRECATED`` error must also be triggered to help people with
 the migration starting one or two minor versions before the version where the
 feature will be removed (depending on the criticality of the removal)::
 
-    trigger_error(
-        'XXX() is deprecated since version 2.X and will be removed in 2.Y. Use XXX instead.',
-        E_USER_DEPRECATED
-    );
+    trigger_error('XXX() is deprecated since version 2.X and will be removed in 2.Y. Use XXX instead.', E_USER_DEPRECATED);