Fix ICMP perf min/max legend values (librenms#18579)

fixes librenms#18574

Author: murrant

includes/html/graphs/device/icmp_perf.inc.php

@@ -57,8 +57,8 @@
 // Average RTT and legend
 $rrd_options[] = "LINE2:ping$line_color:RTT";
 $rrd_options[] = 'GPRINT:ping:LAST:%15.2lf';
-$rrd_options[] = 'GPRINT:min:LAST:%6.2lf';
-$rrd_options[] = 'GPRINT:max:LAST:%6.2lf';
+$rrd_options[] = 'GPRINT:min:MIN:%6.2lf';
+$rrd_options[] = 'GPRINT:max:MAX:%6.2lf';
 $rrd_options[] = 'GPRINT:ping:AVERAGE:%6.2lf\\n';
 
 // loss line and legend