Revert "[Yaml] fixed typo"

This reverts commit 51d1948327227f4064f81cf61f5659e97cb929da.

See: https://github.com/symfony/symfony/commit/51d1948327227f4064f81cf61f5659e97cb929da#commitcomment-2507801 for more information.

This has been detected on the PropelBundle, and Propel2.