Fix a typo: 2^16 -> 2^{16}.

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

protocol/protocol.tex

@@ -11952,7 +11952,7 @@ Several fields are reordered and/or renamed relative to prior versions.
   \presaplingitem{The encoded size of the \transaction \MUST be less than or equal to
         $100000$ bytes.}
   \nufiveonwarditem{\nSpendsSapling, \nOutputsSapling, and \nActionsOrchard{} \MUST all be less
-        than $2^16$.}
+        than $2^{16}$.}
   \presaplingitem{If $\effectiveVersion = 1$ or $\nJoinSplit = 0$, then both \txInCount{} and \txOutCount{} \MUST be nonzero.\!}
   \saplingonwarditem{If $\effectiveVersion < 5$, then at least one of \txInCount, \nSpendsSapling, and \nJoinSplit{} \MUST be nonzero.}
   \saplingonwarditem{If $\effectiveVersion < 5$, then at least one of \txOutCount, \nOutputsSapling, and \nJoinSplit{} \MUST be nonzero.}