我跟着this tutorial构建了推送通知功能。当我尝试以下行时：
openssl s_client -connect gateway.sandbox.push.apple.com:2195 -cert PushChatCert.pem -key PushChatKey.pem
它说：
unable to load Private Key
140735216849372:error:0906D06C:PEM routines:PEM_read_bio:no start line:pem_lib.c:696:Expecting: ANY PRIVATE KEY
我需要做什么？