[S02] equivalent Rational for rat64 should have unsigned denominator.

Just like every other Rat and rat in Perl 6.

S02-bits.pod

@@ -741,7 +741,7 @@ not to the same size.  The numerator should in general be twice the
 size of the denominator to support user expectations.  For instance,
 a C<rat8> actually supports C<Rational[int16,uint8]>, allowing
 numbers like C<100.01> to be represented, and a C<rat64>,
-defined as C<Rational[int128,int64]>, can hold the number of seconds since
+defined as C<Rational[int128,uint64]>, can hold the number of seconds since
 the Big Bang with attosecond precision.  Though perhaps not with
 attosecond accuracy...)