mergesort example

[andriiheonia]

All algorithms are covered by example, except mergesort (it looks like in mergesort it has been missed after refactoring #39 by @secrettriangle ). https://mgechev.github.io/javascript-algorithms/module-sorting_mergesort.html

Also small enhacements:

• type names of each parameter in all documentation begins from uppercase;
• description of each parameter contains "." in the end.

[mgechev]

Yes, makes sense. I'm going to fix this today.