docs: 完善文档

docs/WechatTenpayV3/README.md

@@ -45,7 +45,7 @@ var options = new WechatTenpayClientOptions()
     MerchantId = "微信商户号",
     MerchantV3Secret = "微信商户 v3 API 密钥",
     MerchantCertificateSerialNumber = "微信商户证书序列号",
-    MerchantCertificatePrivateKey = "-----BEGIN PRIVATE KEY-----微信商户证书私钥，即 `apiclient_key.pem` 文件内容-----END PRIVATE KEY-----",
+    MerchantCertificatePrivateKey = System.IO.File.ReadAllText("/微信商户证书私钥文件路径/apiclient_key.pem"),
     PlatformCertificateManager = manager // 平台证书管理器的具体用法请参阅下文的基础用法与加密、验签有关的章节
 };
 var client = WechatTenpayClientBuilder.Create(options).Build();