Docs: add hexa export example for private key

docs/index.md

@@ -53,6 +53,8 @@ var privateKey = new bitcore.PrivateKey();
 var exported = privateKey.toWIF();
 // e.g. L3T1s1TYP9oyhHpXgkyLoJFGniEgkv2Jhi138d7R2yJ9F4QdDU2m
 var imported = bitcore.PrivateKey.fromWIF(exported);
+var hexa = exported.toString();
+// e.g. 'b9de6e778fe92aa7edb69395556f843f1dce0448350112e14906efc2a80fa61a'
 ```
 
 ## Create an Address