Minor correction to what sighashes cover.

Signed-off-by: Daira Hopwood <daira@jacaranda.org>

protocol/protocol.tex

@@ -930,8 +930,8 @@ identified by that previous \joinSplitDescription's $\anchor$.
 \changed{
 \Bitcoin defines several \sighashTypes that cover various parts of a transaction.
 In \Zcash, all of these \sighashTypes are extended to cover the \Zcash-specific
-fields $\nJoinSplit$, $\vJoinSplit$, and $\joinSplitPubKey$. They \emph{do not}
-cover the field $\joinSplitSig$.
+fields $\nJoinSplit$, $\vJoinSplit$, and (if present) $\joinSplitPubKey$.
+They \emph{do not} cover the field $\joinSplitSig$.
 
 \subparagraph{Consensus rule:}
 If $\nJoinSplit > 0$, the \transaction \MUSTNOT use \sighashTypes other than