Improve 02-CMSDataAnalysisSchoolPreExerciseSecondSet.md

Proposed Improvement: In Ex. 7 it is rather confusing if the entire exercise refers to a slimmed file with 1000 events (can be deduced backwards from Ex.8) or to the slimmed file from with all ~50k events. It is especially confusing when being asked to compare original and slimmed file sizes. If the slimmed file contains only 1000 events the comparison is a bit unfair to the full original sample 😄 I propose to add one sentence to make sure the confusion is avoided.

Thank you in advance! See you in June!