ArmPkg/ArmArchTimerLib: correct typos
ArmPkg/ArmArchTimerLib: correct typos

Some minor typographical problems were noticed during previous commits.
This change corrects those, and contains no functional modifications.
The changes are in comments, and one diagnostic message.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Evan Lloyd <cdda240a1142086a2861a2bc5509023c0f3b4e11@arm.com>
Reviewed-by: Ryan Harkin <26c24e1ad4aed8194be813aa696391b1ea0548f2@linaro.org>
Reviewed-by: Ard Biesheuvel <66d3c5fdaeea7ff1f996ad04f2c45e08ab38e2f5@linaro.org>

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