fix: resolve error in help doc of distribution spec flag

[qweeah]

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #1225. Current output after fix

$ oras attach -h
...
 --distribution-spec string                   [Preview] set OCI distribution spec version and API option for target. Options: v1.1-referrers-tag, v1.1-referrers-api
...

$ oras cp -h
...
 --from-distribution-spec string                   [Preview] set OCI distribution spec version and API option for source target. Options: v1.1-referrers-tag, v1.1-referrers-api
...
 --to-distribution-spec string                     [Preview] set OCI distribution spec version and API option for destination target. Options: v1.1-referrers-tag, v1.1-referrers-api
...

Please check the following list:

• Does the affected code have corresponding tests, e.g. unit test, E2E test?
• Does this change require a documentation update?
• Does this introduce breaking changes that would require an announcement or bumping the major version?
• Do all new files have an appropriate license header?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (564bd32) 80.65% compared to head (e970273) 80.59%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1229      +/-   ##
==========================================
- Coverage   80.65%   80.59%   -0.06%     
==========================================
  Files          67       67              
  Lines        3644     3644              
==========================================
- Hits         2939     2937       -2     
- Misses        493      495       +2     
  Partials      212      212              

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[shizhMSFT]

LGTM