chapter 15: missing 'use server' directive in example

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Problem

In the Updating login form step, the action needs to have the 'use server' directive in the beginning of the file.
This is contained in the source code, however the tutorial page does not contain this line. It has led to people getting problems when following the tutorial, as this issue exemplifies.

As the curriculum is currently not open sourced, we cannot edit the examples contained in the tutorial, so I am opening this issue.