minor #12346 [SwiftMailer Spool] Fix double negative (kraynel)

This PR was merged into the 3.4 branch.

Discussion
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[SwiftMailer Spool] Fix double negative

This fixes a double negative related to SwiftMailer file spooling.
I believe that if the cache is cleared, the Message WILL BE unserializable.

Commits
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6d24aa7 [SwiftMailer Spool] Fix double negative

Author: javiereguiluz

email/spool.rst

@@ -153,7 +153,7 @@ interval.
     But when using the filesystem spool, the message class is serialized in
     a file with the randomized class name. The problem is that this random
     class name changes on every cache clear. So if you send a mail and then you
-    clear the cache, the message will not be unserializable.
+    clear the cache, the message will be unserializable.
 
     On the next execution of ``swiftmailer:spool:send`` an error will raise because
     the class ``Swift_Message_<someRandomCharacters>`` doesn't exist (anymore).