usec_per_read_op is measured in microseconds. the formula is incorrect.

https://github.com/newrelic/entity-definitions/blob/0cf81c37718443431ba9462e1cf351127655131d/entity-types/ext-pure/dashboard.json#L163

the formula should be: 
```
latest(usec_per_read_op)/1000
latest(usec_per_write_op)/1000
```
rather than  
```
latest(usec_per_read_op)/1024
latest(usec_per_write_op)/1024
```
