minor #387 Fixed documentation example of @Cache ETag field (o)
minor #387 Fixed documentation example of @Cache ETag field (o)

This PR was squashed before being merged into the 3.0.x-dev branch (closes #387).

Discussion
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Fixed documentation example of @Cache ETag field

Hello!

Currently i'm working with Symfony 2.7 and tried to use @Cache example from documentation. Current example doesn't work.

`ETag` field on annotation used with concat and Expression language expects string for concat operation, but example written for using with `\DateTime` object. (`setLastModifed` already expects `\DateTime` object)

With current example i'm getting `Catchable Fatal Error: Object of class DateTime could not be converted to string`. So i changed example with working version.

Commits
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aafff55 Fixed documentation example of @Cache ETag field

chalasr/SensioFrameworkExtraBundle,sensiolabs/SensioFrameworkExtraBundle,chalasr/SensioFrameworkExtraBundle