Fix bug in galaxy.xsd

[kostrykin]

There is a bug in the XSD introduced in #17581:

The assertion tag <image_has_mean_intensity> is misnamed, and should be corrected to <has_image_mean_intensity>.

How to test the changes?

(Select all options that apply)

• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
• Instructions for manual testing are as follows:
  1. [add testing steps and prerequisites here if you didn't write automated tests covering all your changes]

License

• I agree to license these and all my past contributions to the core galaxy codebase under the MIT license.

[davelopez]

Thank you!