Cache needs be cleared

The configuration is cached, so running assetic:dump before clearing the cache won't use the new configuration.
Update override_dir_structure.rst

Added new line to make code-block work
minor #4219 Cache needs be cleared (burki94)

This PR was submitted for the 2.5 branch but it was merged into the 2.3 branch instead (closes #4219).

Discussion
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Cache needs be cleared

The configuration is cached, so running assetic:dump before clearing the cache won't use the new configuration.

Commits
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7c2c17f Update override_dir_structure.rst
dee483f Cache needs be cleared