Add missing comma to wtperf monitor header

The monitor file outputted by wtperf contains an apparent typo in the header which causes an issue when trying to load it as a csv.

The current line outputted is
#time,totalsec,insert ops per second,modify ops per second,read ops per second,update ops per second,checkpoints,scans,insert average latency(uS),insert min latency(uS),insert maximum latency(uS),modify average latency(uS),modify min latency(uS),modify maximum latency(uS)read average latency(uS),read minimum latency(uS),read maximum latency(uS),update average latency(uS),update min latency(uS),update maximum latency(uS)
 
and I think it should be
#time,totalsec,insert ops per second,modify ops per second,read ops per second,update ops per second,checkpoints,scans,insert average latency(uS),insert min latency(uS),insert maximum latency(uS),modify average latency(uS),modify min latency(uS),modify maximum latency(uS),read average latency(uS),read minimum latency(uS),read maximum latency(uS),update average latency(uS),update min latency(uS),update maximum latency(uS)
 

It appears to be caused by line 1091 in wtperf.c which I believe should have a comma at the end of the string like other similar lines.
{{ "modify maximum latency(uS)"}}

I'm not sure what the guidelines for contributing are but I've made the change (consisting of a single comma) here
https://github.com/JacksonDagger/wiredtiger/tree/monitor_comma