Remove hypenated formatting from set of accepted digest algorithms

[willmhowes]

Given the accepted community standards, I don't believe we should accept hyphenated cases

SRC: https://github.com/iipc/warc-specifications/blob/master/specifications/warc-format/warc-1.1-annotated/index.md#warc-block-digest

Originally posted by @willmhowes in e7fd7b4