Fixed #8234: Corrected typo in docs/cache.txt

Order of MIDDLEWARE_CLASSES

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If you use caching middlewaare, it's important to put each half in the right

If you use caching middleware, it's important to put each half in the right

place within the ``MIDDLEWARE_CLASSES`` setting. That's because the cache

middleware needs to know which headers by which to vary the cache storage.

Middleware always adds something to the ``Vary`` response header when it can.