Fix wrong syntax for the string argument to ValueError

[lilisako]

Description

#5366 (comment)

• In my PR in fix wrong error for metric.structural_similarity when image is too small #5395, I used wrong syntax for the string argument to ValueError.
  Thanks @Pomax for pointing out the issue.

Checklist

• Docstrings for all functions
• Gallery example in ./doc/examples (new features only)
• Benchmark in ./benchmarks, if your changes aren't covered by an
  existing benchmark
• Unit tests
• Clean style in the spirit of PEP8

For reviewers

• Check that the PR title is short, concise, and will make sense 1 year
  later.
• Check that new functions are imported in corresponding __init__.py.
• Check that new features, API changes, and deprecations are mentioned in
  doc/release/release_dev.rst.

[mkcor]

Thank you for your responsiveness! Since we're at it, could you also replace the double quotes with single quotes? Reference: yesterday's discussion starting with #5478 (comment)

[alexdesiqueira]

Thank you @lilisako! Sorry I didn't get this before.