Fix recipe path in `recipe_perfmetrics.rst`

[TomasTorsvik]

Fix typo in documentation for recipe name: recipe_perfmetrics_CMIP5_cds.yml -> recipe_perfmetrics_CMIP5_4cds.yml

Description

• Link to documentation: https://docs.esmvaltool.org/en/latest/recipes/recipe_perfmetrics.html

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[TomasTorsvik]

@ESMValGroup/esmvaltool-coreteam - I need someone to merge this, as I cannot do it myself.