Fix Rat parametarization

(even tho rakudo doesn't yet implement it as a native type, it's
better than a type that doesn't exist)
rakudo/rakudo@cf15435575
rakudo/rakudo@6977680bd9

Author: zoffixznet

doc/Type/Rat.pod6

@@ -4,7 +4,7 @@
 
 =SUBTITLE Rational number (limited-precision)
 
-    class Rat is Cool does Rational[Int, UInt64] { }
+    class Rat is Cool does Rational[Int, uint64] { }
 
 C<Rat> objects store rational numbers as a pair of a numerator and
 denominator. Number literals with a dot but without exponent produce