Hash prefix in base64 excludes the "u" character that HoloHashB64 prepends

## What is incorrect
The base64 hash prefixes are correct, but missing the "u" character that HoloHashB64 expects.

This is explained in the rust docs page: https://docs.rs/hdi/latest/hdi/hash/index.html#self-identifying-hash-encoding

My suggestion is to include that rust docs page in the "References" section.

## Link to page
https://developer.holochain.org/build/identifiers

Start with the documentation page linked in the issue and locate its References section. Add the linked hdi Rust documentation page there, then verify that the reference explains the leading "u" expected by HoloHashB64.

Written by the indexing model from the issue text.