[zh-CN]change identity.md for better understanding

[Pheglovog]

some translations in "key concept/identity" was obscure, so that I make some little but good change to make translations more easily to understand

A PKI is comprised of Certificate Authorities who issue digital certificates to parties (e.g., users of a service, service provider), who then use them to authenticate themselves in the messages they exchange in their environment。

As a result, if one trusts the CA (and knows its public key), it can trust that the specific actor is bound to the public key included in the certificate, and owns the included attributes, by validating the CA’s signature on the actor’s certificate._ 这些地方比较难懂，在“关键概念/身份”这一部分，英语中长句子使用机翻读起来太难懂了，我想把这部分修改一下