(Membership Information): Fixed typo: "added-by", not "added-byp".

doc/Protocol-A.texi

@@ -2319,7 +2319,7 @@ the data anyhow.
 Additional texts beyond @field{last-text-read} that have also been
 read.  This will be empty if @field{unread-is-secret} is set, unless
 you have access to the data anyhow.
-@item added-byp
+@item added-by
 The person who created the membership.  This field is zero if the
 membership was created before protocol version 10 was introduced.
 @item added-at