Fix grammar.

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

@@ -245,7 +245,7 @@ Keeping the members of the groups B, C, and D up-to-date
 At least every 2 years, we make sure that the mechanism still works:
 
 1. We review internally the list of members of each group and decide
-   possible additions and removals from each group.
+   possible additions to, and removals from, each group.
 
 1. We write to the list to ask all members to check that they still have
    their share and the number in the file name.