EmbeddedPkg/MetronomeDxe: Update outdated lower bound comment

EmbeddedPkg/MetronomeDxe: Update outdated lower bound comment

There was previously a lower bound on the value of TickPeriod such that
it couldn't be less than 10 us. However, that was removed from the PI
Specification in the 1.0 errata released in 2007. From the revision
history:

"M171 Remove 10 us lower bound restriction for the TickPeriod in the
Metronome"

Update the documentation of TickPeriod in MetronomeDxe/Metronome.c to
remove mention of the lower bound.

Signed-off-by: Rebecca Cran <rebecca@quicinc.com>
Reviewed-by: Leif Lindholm <quic_llindhol@quicinc.com>