[atlas] [NTP measurements] Minimum offset if negative

Measurement #74488034 says on <https://atlas.ripe.net/measurements/74488034>: "Min Offset: -1 173,799". Since offsets can be positive or negative, I suggest it is not a good idea to take the minimum without first removing the sign. Here, the real minimum offset was -0.000345 (the closest from the server) and the maximum -1.173799 (the farthest from the server). (There is also a strange formatting on the Web, and a lack of units.)