fix expected_update_rate_--->minimum_update_interval_

[cf-zhang]

expected update rate is HZ , and the meaning in isCurrent() is Time Duration.

[cf-zhang]

expected update rate is HZ , and the meaning in isCurrent() is Time Duration.

~<name>/<source_name>/expected_update_rate (double, default: 0.0)

    How often to expect a reading from a sensor in seconds. A value of 0.0 will allow infinite time between 
readings. This parameter is used as a failsafe to keep the navigation stack from commanding the robot 
when a sensor has failed. It should be set to a value that is slightly more permissive than the actual rate of 
the sensor. For example, if we expect a scan from a laser every 0.05 seconds we might set this parameter 
to be 0.1 seconds to give a generous buffer and account for some amount of system latency. 

I went to wiki check it again, the description for expected_update_rate is namely the time Duration. I'm sorry abort this pull request, but I still think the name of this parameter is ambiguous.