Update range docs: microseconds as supported units

[jmargeta]

This updates the docs to reflect that microseconds as units in range work just fine.

import pendulum

start = pendulum.datetime(2000, 1, 1, microsecond=1, tz='Europe/Bratislava')  
end = pendulum.datetime(2000, 1, 1, microsecond=42, tz='Europe/Bratislava') 

period = pendulum.period(start, end) 
for dt in period.range('microseconds', 4):
    print(dt) 

Output

2000-01-01T00:00:00.000001+01:00
2000-01-01T00:00:00.000005+01:00
2000-01-01T00:00:00.000009+01:00
2000-01-01T00:00:00.000013+01:00
2000-01-01T00:00:00.000017+01:00
2000-01-01T00:00:00.000021+01:00
2000-01-01T00:00:00.000025+01:00
2000-01-01T00:00:00.000029+01:00
2000-01-01T00:00:00.000033+01:00
2000-01-01T00:00:00.000037+01:00
2000-01-01T00:00:00.000041+01:00