[Parcel] Add 1-element Parcel example.

Author: pmichaud

lib/Parcel.pod

@@ -11,6 +11,7 @@ C<()>, it is really the comma that create a Parcel.
     ()      # empty Parcel
     (1 + 2) # not a Parcel
     (1, 2)  # Parcel with two elements
+    (1,)    # Parcel with one element
 
 Parcels are immutable, but can contain mutable containers.