Fix iam.martin_ruiz equation in docs #1561

[echedey-ls]

• Closes Equation in docstring of pvlib.iam.martin_ruiz does not match code #1561
• I am familiar with the contributing guidelines
• Adds description and name entries in the appropriate "what's new" file in docs/sphinx/source/whatsnew for all changes. Includes link to the GitHub Issue with :issue:`num` or this Pull Request with :pull:`num`. Includes contributor name and/or GitHub username (link with :ghuser:`user`).
• Pull request is nearly complete and ready for detailed review.
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Does not apply:

• Tests added
• Updates entries in docs/sphinx/source/reference for API changes.
• New code is fully documented. Includes numpydoc compliant docstrings, examples, and comments where necessary.

Description of issue in #1561
First time contributor here! Looking forward to make bigger PRs in short, so all tips and criticism is welcomed.

[echedey-ls]

I've got a doubt in the optional use of ~ in :py:func:`~pvlib.iam.martin_ruiz`, I've seen some rST entries don't use it (I didn't) but all still works. Can you tell me if it's use is recommended or not?

[kandersolar]

Looks great, thanks @echedey-ls!

I've got a doubt in the optional use of ~ in :py:func:~pvlib.iam.martin_ruiz, I've seen some rST entries don't use it (I didn't) but all still works. Can you tell me if it's use is recommended or not?

The ~ controls whether the HTML link text will look like pvlib.iam.martin_ruiz or just martin_ruiz. See https://www.sphinx-doc.org/en/master/usage/restructuredtext/domains.html#cross-referencing-syntax

Sometimes it makes sense to use ~ and sometimes it's better not to use it. For this PR I would leave it the way it is now, but it's really just down to personal taste.

[echedey-ls]

Thanks for the reference @kanderso-nrel!

[kandersolar]

Looking forward to make bigger PRs in short

Please let us know if we can help :) and thanks again for this PR!