Minor bug fix #144

Easylogging++ v9.50 RELEASE NOTES
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=     BUG FIXES       =
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 - Issue with datetime format (%y and %Y having same behaviour) (issue #144)

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=         NOTES          =
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 - See https://github.com/easylogging/easyloggingpp/tree/master/doc for other release notes
 - Closed issues: https://github.com/easylogging/easyloggingpp/issues?page=1&state=closed