Sensor doc: nxt-analog-host to lego-port class

Per update at 3.16.1-8-ev3dev  
"*host" drivers for input ports removed and merged functionality into the "lego-port" class. 
Link to http://www.ev3dev.org/docs/drivers/nxt-analog-host/ is no longer valid.
Merge pull request #36 from BertLindeman/patch-2

Sensor doc: nxt-analog-host to lego-port class