fix a typo in the signing key revocation mechanism

wiki/src/doc/about/openpgp_keys/signing_key_revocation.mdwn

@@ -19,8 +19,8 @@ Covered by current proposal:
      becomes unavailable.
   E. People not from tails@boum.org shouldn't know how the shares are spread
      and who has them.
-  F. People in possession of a share of the signing key should have
-     instructions on how to use it if needed.
+  F. People in possession of a share of the revocation certificate
+     of the signing key should have instructions on how to use it if needed.
 
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