doc/job_traces.md: the file is job.log

What does this MR do?

Fix documentation in https://docs.gitlab.com/ce/administration/job_traces.html#data-flow

In my system the files are named job.log not trace.log:

find /var/opt/gitlab/gitlab-rails/shared/artifacts -type f | grep -F .log|awk -F/ '{print $NF}'|sort |uniq -c|tail -n10
      1 194525.log
      1 194526.log
      1 194527.log
      1 194528.log
      1 194529.log
      1 194530.log
      1 194531.log
      1 194533.log
      1 194536.log
 241396 job.log