Fixed: API Exception Error format

src/Lidarr.Http/Exceptions/ApiException.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using Nancy;
 using Nancy.Responses;
 using Lidarr.Http.ErrorManagement;
@@ -30,10 +30,10 @@ namespace Lidarr.Http.Exceptions
 
             if (content != null)
             {
-                result = result + " :" + content;
+                result = $"{result}: {content}";
             }
 
             return result;
         }
     }
-}
+}