Update zh-Hans.lproj/NSDateTimeAgo.strings

1.Modified some translation, to make it more suitable/understandable for Chinese users.
2.According to Chinese Reading Habit: Usually, there is no "space" between characters.
For example:
We usually take "1 day ago" as "1天前", not "1 天前". The space after "1" feels a little uncomfortable/machine-generated. "1天前" feels more natural:)