misc: Fix spelling for SesssionExemptMixin

[dkalamar]

• Fixes spelling error for class SesssionExemptMixin to SessionExemptMixin

[boring-cyborg]

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[hussein-awala]

@eladkal @ephraimbuddy IMHO this change is safe because the class is not part of the public API, could you check it?

[boring-cyborg]

Awesome work, congrats on your first merged pull request! You are invited to check our Issue Tracker for additional contributions.