secure_distribution has wrong type (#462)

If I am following https://cloudinary.com/documentation/cloudinary_sdks#configuration_parameters correctly the type should be string. But it seems that a boolean is acceptable. Changing to string to see if someone can take a look if this is accurate. I should be able to place my cname in here.

Author: Alex Patterson

types/index.d.ts

@@ -334,7 +334,7 @@ declare module 'cloudinary' {
         api_key?: string;
         api_secret?: string;
         private_cdn?: boolean;
-        secure_distribution?: boolean;
+        secure_distribution?: string;
         force_version?: boolean;
         ssl_detected?: boolean;
         secure?: boolean;