Update ATT&CK coverage URL(s) in docs/ATT&CK-coverage.md (#4091)

* Update ATT&CK coverage URL(s) in docs/ATT&CK-coverage.md

* Update docs/ATT&CK-coverage.md

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Co-authored-by: shashank-elastic <91139415+shashank-elastic@users.noreply.github.com>

(cherry picked from commit fab842b)