docfix postcondition of symmetricPower

According to the specification of symmetricRemainder in si.spad

   symmetricRemainder     : (%, %) -> %
      ++ symmetricRemainder(a, b) (where \spad{b > 1}) yields r
      ++ where \spad{-b/2 <= r < b/2}.

the result of
```
%%% (1) -> symmetricRemainder(2,4)

   (1)  2
```
is clearly wrong (should be -2 according to the above specification).
This patch corrects the specification.

src/algebra/si.spad

@@ -31,7 +31,7 @@ IntegerNumberSystem() : Category ==
       ++ where \spad{0 <= r < b} and \spad{r = a rem b}.
    symmetricRemainder     : (%, %) -> %
       ++ symmetricRemainder(a, b) (where \spad{b > 1}) yields r
-      ++ where \spad{-b/2 <= r < b/2}.
+      ++ where \spad{-b/2 < r <= b/2}.
    rational? : % -> Boolean
       ++ rational?(n) tests if n is a rational number
       ++ (see \spadtype{Fraction Integer}).