Update chapters/05-exercise-2.md

Conclusion about how to retrieve posted json enconded parameters in the server-side third party API
removed none required 'a'
Merge pull request #306 from steriley/patch-1

Removed 'a'
Merge pull request #305 from anibalsanchez/patch-2

Minor edit to add a conclusion about how to get the parameters in the server
Rewrote chapter 5
Added the code from chapter 5 to the repo