Fix broken ref to board machine names page

[kprosise]

Added space, link is now in the generated HTML as intended.

QA: checked rendered output and link. Ran linkcheck. No reason to run linter.

This commit addresses FS-2843, re: fix see:ref:ref-linux-supported

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Checklist

• Run spelling and grammar check, preferably with linter.
• Avoid changing any header associated with a link/reference.
• Match tone and style of page/section.
• Run make linkcheck.
• View HTML in a browser to check rendering.
• Use semantic newlines.
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[doanac]

Docs for dcf5738 are browsable at: https://ci.foundries.io/projects/fio-docs/builds/2802/docs/artifacts/html/index.html

[Tim-Anderson]

LGTM

[vanmaegima]

@kprosise this should be good to merge so I can close the customer ticket.