Add doc example for `CString::as_c_str`.
Add doc example for `CString::as_c_str`.

aidancully/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,graydon/rust,graydon/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,aidancully/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,graydon/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,graydon/rust