Removed wrong description of option in portrait plot docstring

[schlunma]

Description

While copy/pasting a docstring from another diagnostic, I messed up the description of matplotlib_rc_params. This PR fixes this.

• Link to documentation: https://esmvaltool--3908.org.readthedocs.build/en/3908/api/esmvaltool.diag_scripts.portrait_plot.html#configuration-parameters-through-recipe

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