doc: Reworded "cert" and "key" explanation
doc: Reworded "cert" and "key" explanation

"cert" and "key" wording uses the same structure as "ca" wording.

Credit: da39a3ee5e6b4b0d3255bfef95601890afd80709@rvedotrc
PR-URL: https://github.com/npm/npm/pull/12415

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