minor #14916 Update conventions page to show correct function (dayallnash)

This PR was merged into the 5.2 branch.

Discussion
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Update conventions page to show correct function

* Updated conventions page to say trigger_deprecation instead of trigger_error

Commits
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1bb5c08 Update conventions page to show correct function

Author: OskarStark

contributing/code/conventions.rst

@@ -167,7 +167,7 @@ A deprecation must also be triggered to help people with the migration
 
     trigger_deprecation('symfony/package-name', '5.1', 'The "%s" class is deprecated, use "%s" instead.', Deprecated::class, Replacement::class);
 
-When deprecating a whole class the ``trigger_error()`` call should be placed
+When deprecating a whole class the ``trigger_deprecation()`` call should be placed
 after the use declarations, like in this example from `ServiceRouterLoader`_::
 
     namespace Symfony\Component\Routing\Loader\DependencyInjection;