Update cache.rst
Update cache.rst

The value for the `public` attribute should be specified without quotation marks. For the construction `public="false"`, actually `public` takes the value `true`, because a string converted to Boolean will always be `true`.
henrikbjorn/ParamConverter,chalasr/SensioFrameworkExtraBundle,chalasr/SensioFrameworkExtraBundle,sensiolabs/SensioFrameworkExtraBundle