check_graphite_threshold - fix wrong usage info

the examples here are wrong somehow. if you actually
run it with "10m" for example, it says:

UNKNOWN: execution of the check script exited with exception

The unit specification for --from should be
one of the following: hours,h,w,weeks,min

so that does not say "m", but it does say "h" and "w" ?;p

Change-Id: I0fc2c7bf97806b26fcec59949fe66e1fa7a2b973