rephrase presentation of the input file

Spurious references about graphs, x-values, and y-values are removed because they are not connected at all with the subject.
The term "statistic population" is introduced, from which it can be inferred that the program should calculate the population variance (and not the sample variance)
should fix issue #1618

subjects/math-skills/README.md

@@ -23,7 +23,7 @@ Your program must be able to read from a file and print the result of each stati
 ...
 ```
 
-This data represents a graph in which the values of the x axis are the number of the lines (0, 1, 2, 3, 4, 5 ...) and the values of the y axis are the actual numbers (189, 113, 121, 114, 145, 110...).
+This data represents a [statistical population](https://en.wikipedia.org/wiki/Statistical_population): each line contains one value.
 
 To run your program a command similar to this one will be used if your project is made in Go: