Fix codos in pick signatures in Bool.rakudoc (#4365)

Dunno if I should just be committing changes like this...

Author: raiph

doc/Type/Bool.rakudoc

@@ -78,8 +78,8 @@ and any key.
 
 =head2 routine pick
 
-    multi method pick(Bool:U --> Bool:D)
-    multi method pick(Bool:U $count --> Seq:D)
+    multi method pick(Bool:U: --> Bool:D)
+    multi method pick(Bool:U: $count --> Seq:D)
 
 Returns C<True> or C<False> if called without any argument. Otherwise returns
 C<$count> elements chosen at random (without repetition) from the C<enum>. If