update Admin.NET/Admin.NET.Core/Service/Auth/SysAuthService.cs.

修正验证码的codeId为较长的long值时，返回前端会丢失精度，造成验证失败

Signed-off-by: ileego <fei.gao@live.cn>

Admin.NET/Admin.NET.Core/Service/Auth/SysAuthService.cs

@@ -219,8 +219,8 @@ public class SysAuthService : IDynamicApiController, ITransient
     [DisplayName("获取验证码")]
     public dynamic GetCaptcha()
     {
-        var codeId = YitIdHelper.NextId();
-        var captcha = _captcha.Generate(codeId.ToString());
+        var codeId = YitIdHelper.NextId().ToString();
+        var captcha = _captcha.Generate(codeId);
         return new { Id = codeId, Img = captcha.Base64 };
     }