Update deployment.rst

[proArtex]

Looks like SYMFONY_ENV is no longer exists (flex + framework-bundle). It is now APP_ENV

[wouterj]

Hmm, this is a tricky one. When using Symfony 3.3 in the Flex edition, you're 100% correct (ref). However, when using Symfony 3.3 with the Standard Edition (old way) it's still called SYMFONY_ENV (ref).

Let's see what other people think about this. Maybe we should include both and indicate when to use which one (e.g. "run export SYMFONY_ENV=prod (or export APP_ENV=prod if you're using Symfony Flex) before running this command so that"). But I'm not sure if that doesn't make stuff too verbose.

[proArtex]

Maybe better fork documentation for Flex? I was confused for example when tried to override twig error templates with Flex (no Resources folder exists).

[javiereguiluz]

Thank you @proArtex.