Rephrase cmake comment about CMP0077

[scpeters]

🦟 Bug fix

Improves a cmake comment

Summary

While updating cmake logic as part of gazebosim/gz-cmake#350, I noticed a typo in the cmake version in a comment about setting policy CMP0077 (13.13 instead of 3.13), so I've fixed the typo and also rephrased the comment to refer to the cmake_minimum_version instead of the cmake version, since the policy would have no effect on old versions of cmake; it only affects newer versions of cmake while the cmake_minimum_version is set to a version older than 3.13.

cc @Blast545

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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