minor symfony#4738 [Contributing][Code] update year in license (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
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[Contributing][Code] update year in license

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | no
| Applies to    | all
| Fixed tickets |

Commits
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f1345b2 [Contributing][Code] update year in license

Author: wouterj

contributing/code/license.rst

@@ -16,7 +16,7 @@ According to `Wikipedia`_:
 The License
 -----------
 
-Copyright (c) 2004-2013 Fabien Potencier
+Copyright (c) 2004-2015 Fabien Potencier
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal