RFC6: Use contribution guidelines for changes to CoC

rfcs/0006-code-of-conduct.rst:
Change the Specification section to point at the Service Agreement
repository contribution guidelines in regards to how to change the
document instead of reiterating the current contribution guidelines
(which are subject to change).
1 job for code-of-conduct in 13 seconds (queued for 7 seconds)
Status Name Job ID Coverage
  Lint
passed rstcheck #34527

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