fix: create order request example amount field incorrect type

[thanhtinhpas1]

I find out that the example CreateOrderRequest has a field number which wrong type, number instead of string. So this MR will fix that.

Note: I test this by code as the image below

Please take a look anh @anhldbk
IMHO if amount field goes along with currency or describes the default currency as VND will be better.

[anhldbk]

@leonthaibao PTAL

[leonthaibao]

Thanks @thanhtinhpas1 for your contribution.
About adding more description to the example of the readme. Because the default currency is VND so we don't have to add the currency field and the document should have precise detail on how to use the function not on the SDK readme. So for now, we will link the document to our SDK