minor #6212 Typo in default session save_path (DerekRoth)

This PR was merged into the 2.3 branch.

Discussion
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Typo in default session save_path

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets |

The %kernel.cache.dir% parameter does not exist, %kernel.cache_dir% does.

Commits
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f91a429 Typo in default session save_path

Author: xabbuh

reference/configuration/framework.rst

@@ -800,7 +800,7 @@ start and depends on `gc_divisor`_ and `gc_probability`_.
 save_path
 .........
 
-**type**: ``string`` **default**: ``%kernel.cache.dir%/sessions``
+**type**: ``string`` **default**: ``%kernel.cache_dir%/sessions``
 
 This determines the argument to be passed to the save handler. If you choose
 the default file handler, this is the path where the session files are created.