Check that tutorials run in the latest version of Astropy

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This summer we will check that each tutorial currently available on the Learn Astropy website works in the latest version of Astropy. Use this issue as a check-list and report progress.

• Does every Jupyter notebook cell run as expected?
• If there are bugs, were you able to fix them? If not, we will find the best person to contact regarding the issue.

First set of tutorials to check:

• [ ] Modifying a FITS header
• [ ] FITS tables
• [ ] FITS images