Fix Sample input for "Course grading, part 1" exercise

The content of sample students file is not reflected in sample output
of the exercise and we can see that the next exercises also base their
results on the output, so better fix the input to sync up with further
text of the article.

Author: olegresearcher

data/part-6/1-reading-files.md

@@ -572,9 +572,9 @@ This program works with two CSV files. One of them contains information about so
 
 ```csv
 id;first;last
-12345678;peter;pythons
-12345687;jean;javanese
-12345699;alice;adder
+12345678;pekka;peloton
+12345687;jaana;javanainen
+12345699;liisa;virtanen
 ```
 
 The other contains the number of exercises each student has completed each week: