Fix unit constants to be consistent with code

Extend example to show querying different units or specific sensors

lua_modules/ds18b20/ds18b20-example.lua

@@ -13,6 +13,14 @@ end
 -- Just read temperature
 print("Temperature: "..t.read().."'C")
 
+-- Get temperature of first detected sensor in Fahrenheit
+print("Temperature: "..t.read(nil,t.F).."'F")
+
+-- Query the second detected sensor, get temperature in Kelvin
+if (table.getn(addrs) >= 2) then
+   print("Second sensor: "..t.read(addrs[2],t.K).."'K")
+end
+
 -- Don't forget to release it after use
 t = nil
 ds18b20 = nil

lua_modules/ds18b20/ds18b20.lua

@@ -33,9 +33,9 @@ setfenv(1,M)
 --------------------------------------------------------------------------------
 -- Implementation
 --------------------------------------------------------------------------------
-C = 0
-F = 1
-K = 2
+C = 'C'
+F = 'F'
+K = 'K'
 function setup(dq)
   pin = dq
   if(pin == nil) then