std random.rs: update link to RTEMS docs

[DanielEScherzer]

The old URL with master resulted in a 404 error - use main instead.

[rustbot]

r? @Mark-Simulacrum

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The reviewer was selected based on:

• Owners of files modified in this PR: @ChrisDenton, libs
• @ChrisDenton, libs expanded to 8 candidates
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[DanielEScherzer]

@rustbot label +A-docs