Update to Sorting section and Sorting exercise

[zjsteyn]

I updated some of the wording referring to a "1980" sheet in the spreadsheet that was no longer available in the data downloaded at the start of the lesson.

I also included a sorting exercise for consideration based on the example of sorting the weight column and a solution was also provided.

The images displays quite large, I could resize them if required and the exercise is found useful.

[tracykteal]

Thanks, this looks good. I just added a comment about how to link to the excel file for download.

[ErinBecker]

Fixes https://github.com/datacarpentry/spreadsheet-ecology-lesson/issues/119

[tracykteal]

Thanks, nice addition.