Add missing "else" in SSAMOSWAP pseudocode (#1560)

This doesn't change the meaning of the pseudocode because "raise" is presumably
a terminal statement, but it makes the code easier to read.

h/t @tsaiyenting

src/unpriv-cfi.adoc

@@ -773,7 +773,7 @@ data values.
 ----
   if privilege_mode != M && menvcfg.SSE == 0
       raise illegal-instruction exception
-  if S-mode not implemented
+  else if S-mode not implemented
       raise illegal-instruction exception
   else if privilege_mode == U && senvcfg.SSE == 0
       raise illegal-instruction exception
@@ -797,7 +797,7 @@ address in `rs1`.
 ----
   if privilege_mode != M && menvcfg.SSE == 0
       raise illegal-instruction exception
-  if S-mode not implemented
+  else if S-mode not implemented
       raise illegal-instruction exception
   else if privilege_mode == U && senvcfg.SSE == 0
       raise illegal-instruction exception