Fix sha256 return type and encode / decode condition

[KattMingMing]

Description

There were a couple of issues in index.md and in the completed/letter-shop project that caused various errors. Below are an outline of the issues as well as proposed fixes. The main issues were that http://localhost:8000 didn't actually load and that once it was loading payments were not fulfilled. Hope this is helpful.

cc/ @michielbdejong

• Fix loading http://localhost:8000
  • hash.digest([encoding]) returns a Buffer if no encoding is passed and I think what is intended is a base64 encoding.
• Removes unnecessary ./X.js
  • Update documentation for consistency in earlier versions of document
• Fulfillment payment updates
  • incoming_prepare returned URL encoded transfer.exeuctionCondition which caused the fulfillment to be undefined. This happened because letters[fulfillment] was set using fulfillment as a base64url. The transfer.exeuctionCondition then needs to be converted back so that they match.

[adrianhopebailie]

@KattMingMing thanks for that. I obviously messed things up when I was fixing my eslint errors and didn't test properly!