Correct grammar for digest algorithm portion

The current grammar specifies that the characters [A-Fa-f0-9_+.-] are
valid in the "algorithm" portion of a digest. This contradicts the
specification stating that "compliant implementations SHOULD use
sha256", as "sha256" does not match the above rule.

This commit fixes this issue by additionally allowing characters g-zG-Z
in the "algorithm" portion.

Signed-off-by: Fabian Freyer <[email protected]>

spec.md

@@ -234,7 +234,7 @@ We define a _digest_ string to match the following grammar:
 
 ```EBNF
 digest      := algorithm ":" hex
-algorithm   := /[A-Fa-f0-9_+.-]+/
+algorithm   := /[A-Za-z0-9_+.-]+/
 hex         := /[A-Fa-f0-9]+/
 ```