Updated example for makeCross function

[domwaters19]

• Improved the example in the documentation for makeCross to make it more general.
• ids are now supplied as characters in the crossPlan rather than as integer values.
• This handles situations where the position of an individual in pop@id does not align with their id value.

[gregorgorjanc]

@domwaters19 i suggest you expand this example a bit to showcase that we can refer to individuals by integers and names/characters