reference: allow more than 1 digest algorithm separator

[stevvooe]

This updates the grammar to allow more than one digest algorithm
separator, matching the regular expression and intended grammar.

Signed-off-by: Stephen J Day [email protected]

[codecov]

Codecov Report

Merging #2253 into master will decrease coverage by 10.06%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master    #2253       +/-   ##
===========================================
- Coverage   61.61%   51.54%   -10.07%     
===========================================
  Files         125      125               
  Lines       11395    11395               
===========================================
- Hits         7021     5874     -1147     
- Misses       3489     4776     +1287     
+ Partials      885      745      -140

Impacted Files	Coverage Δ
reference/reference.go	80.6% <ø> (ø)	⬆️
registry/storage/driver/gcs/gcs.go	0.4% <0%> (-68.94%)	⬇️
registry/storage/driver/oss/oss.go	0.56% <0%> (-57.23%)	⬇️
registry/storage/driver/s3-aws/s3.go	4.59% <0%> (-56.33%)	⬇️
registry/storage/driver/s3-goamz/s3.go	0.5% <0%> (-50.88%)	⬇️
registry/client/transport/transport.go	69.69% <0%> (-9.1%)	⬇️

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[dmcgowan]

LGTM