Fix percentage performance data for used memory

This changes the label of an optional performance data value
which to the correct string, which is an error resulting from
copy and pasting another section.

cmd/memory.go

@@ -214,7 +214,7 @@ func computeMemResults(config *memory.MemConfig, memStats *memory.Mem) result.Pa
 
 	MemUsedPercentage := float64(memStats.VirtMem.Used) / (float64(memStats.VirtMem.Total) / 100)
 	pdMemUsedPercentage := perfdata.Perfdata{
-		Label: "free_memory_percentage",
+		Label: "used_memory_percentage",
 		Value: MemUsedPercentage,
 		Uom:   "%",
 	}