Update pruning_tutorial.py

[younghoonNa]

Description

The kernel size of the LeNet model is incorrectly defined as 3. It needs to be changed to 5.

Checklist

• [ ] The issue that is being fixed is referred in the description (see above "Fixes #ISSUE_NUMBER")
• [ ] Only one issue is addressed in this pull request
• [ ] Labels from the issue that this PR is fixing are added to this pull request
• [ ] No unnessessary issues are included into this pull request.

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