Greek: Fixed a typo re: size units in messages file.

Because of copy/paste in the previous pull request for 1.1.13, some size units were not translated correctly. This will probably take a while before 1.1.14, but I wouldn't like to leave it as it is. Thanks to olliebennett for letting me know (https://github.com/yiisoft/yii/commit/473e38ec6a375fa9328782d587673b37d273daaf#commitcomment-2390050).