fix doc quotes around parameter.variable

[ninaburg]

Description

Hi dear earthkit-devs,

From the Fieldlist doc:

I obtained NameError: name "parameter" is not defined:

So I added the quotes around parameter.variable in the corresponding doc in this PR.

Feel free to ignore and close the PR if this is supposed to work.

Cheers

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• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.