Fix invalid class referenced in docblock

Change reference to non-existing Swift_Mime_Entity to Swift_Mime_MimeEntity.
minor #540 Fix invalid class referenced in docblock (c960657)

This PR was merged into the 5.3-dev branch.

Discussion
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Fix invalid class referenced in docblock

Swift_Mime_SimpleMimeEntity references the non-existing class Swift_Mime_Entity.

Commits
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8449b52 Fix invalid class referenced in docblock