Add s390x selector missing from docs

[skupr-anaconda]

Description

Add s390x selector missing from docs to fix an issue https://github.com/conda/conda-build/issues/4536

Checklist - did you ...

• [x] Add a file to the news directory (using the template) for the next release's release notes?
• [x ] Update outdated documentation

[conda-bot]

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[jezdez]

@conda-bot check