minor #2598 Update advanced.rst (pfwd)
minor #2598 Update advanced.rst (pfwd)

This PR was submitted for the 2.x branch but it was merged into the 1.x branch instead (closes #2598).

Discussion
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Update advanced.rst

I noticed that the rot13Provider wasn't called correctly as it was trying to access a local variable instead of calling $this

Commits
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b42fd643 Update advanced.rst

nicolas-grekas/Twig,twigphp/Twig,nicolas-grekas/Twig,nicolas-grekas/Twig,nicolas-grekas/Twig,twigphp/Twig,twigphp/Twig,nicolas-grekas/Twig