fix(collection): fixed error message in Subset.

[fredbi]

This fix bug reported at
stretchr#1848.

Our fork suffered from the same issue.

Change type

Please select: 🆕 New feature or enhancement|🔧 Bug fix'|📃 Documentation update

Short description

Fixes

Full description

Checklist

• I have signed all my commits with my name and email (see DCO. This does not require a PGP-signed commit
• I have rebased and squashed my work, so only one commit remains
• I have added tests to cover my changes.
• I have properly enriched go doc comments in code.
• I have properly documented any breaking change.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.32%. Comparing base (16f6c61) to head (59b8553).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #101      +/-   ##
==========================================
- Coverage   91.34%   91.32%   -0.03%     
==========================================
  Files          94       94              
  Lines       12226    12226              
==========================================
- Hits        11168    11165       -3     
- Misses        838      840       +2     
- Partials      220      221       +1     

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