minor symfony#10165 fix cache option data type (xabbuh)

This PR was merged into the 2.8 branch.

Discussion
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fix cache option data type

see symfony#10102 (comment)

Commits
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607d23a fix cache option data type

Author: javiereguiluz

reference/configuration/twig.rst

@@ -121,7 +121,7 @@ application harder to maintain.
 cache
 ~~~~~
 
-**type**: ``string`` | ``false`` | ``Twig\Cache\CacheInterface`` **default**: ``'%kernel.cache_dir%/twig'``
+**type**: ``string`` | ``false`` **default**: ``'%kernel.cache_dir%/twig'``
 
 Before using the Twig templates to render some contents, they are compiled into
 regular PHP code. Compilation is a costly process, so the result is cached in