l10n: Set nplurals of zh_CN.po from 1 to 2

l10n: Set nplurals of zh_CN.po from 1 to 2

In most cases, plural-forms are unnecessary for Chinese. For example,
"apple" and "apples" are the same in Chinese, they are both translated
as "苹果". While there are exceptions, e.g., the plural form of "he",
"she" and "it" is "they" in English. In Chinese, "他(he)", "她(she)",
and "它(it)" have plural forms too, they are "他们", "她们", and "它们".

But what makes 'nplurals=1' hard to work right for Chinese is:

    #: bundle.c:192
    #, c-format
    msgid "The bundle requires this ref"
    msgid_plural "The bundle requires these %d refs"

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>