Fixed wrong variable in Franke3D function

[Timo-Schrader]

Main changes of this PR

This PR fixes one wrong variable which is encoded in the 3D Franke function. It changes the variable y to z.

Author's checklist

• [ ] I used the pre-commit hook and used pre-commit run --all to apply all available hooks.
• [ ] I added a test to cover the proposed changes in our test suite.
• [ ] I updated the documentation in docs/README.md.
• [ ] I updated potential breaking changes in the tutorial precice/tutorials/aste-turbine.

[kursatyurt]

LGTM.

[davidscn]

According to the function definition found here: http://www.raczar.es/webracz/ImageServlet?mod=publicaciones&subMod=monografias&car=monografia20&archivo=Monogr20-8Bozzini.pdf