fix(mem): Correct capitalization of linux writeback

fixes: #1471

Author: powersj

mem/mem_linux.go

@@ -154,13 +154,13 @@ func fillFromMeminfoWithContext() (*VirtualMemoryStat, *VirtualMemoryExStat, err
 				return ret, retEx, err
 			}
 			retEx.Unevictable = t * 1024
-		case "WriteBack":
+		case "Writeback":
 			t, err := strconv.ParseUint(value, 10, 64)
 			if err != nil {
 				return ret, retEx, err
 			}
 			ret.WriteBack = t * 1024
-		case "WriteBackTmp":
+		case "WritebackTmp":
 			t, err := strconv.ParseUint(value, 10, 64)
 			if err != nil {
 				return ret, retEx, err