Clarify that the requirement is to "use" a key, not possess it.

Co-authored-by: Dave Longley <[email protected]>

index.html

@@ -621,7 +621,7 @@ <h3>Terminology</h3>
 A method and its parameters used to independently verify a proof. For
 example, a cryptographic public key can be used as a verification method with
 respect to a digital signature; in such use, it verifies that the signer
-possessed the associated cryptographic private key.
+used the associated cryptographic private key.
             </p>
             <p>
 "Verification" and "proof" in this definition are intended to apply broadly. For