Fix bug in SA statement: desc() applied to column, not statement

[jdavcs]

desc() is applied to the column object, not the statement.

How to test the changes?

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• 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]

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

[martenson]

Thanks John!