Fix Typo on Occurrences

[jeffhall-wk]

Hello!

I was recently running into the following error when my java cron would try to record a failure, and it would completely drop my worker from locust master.

$ /ERROR/stderr: Traceback (most recent call last):
$ /ERROR/stderr:
$ /ERROR/stderr: File "src/gevent/greenlet.py", line 854, in gevent._greenlet.Greenlet.run
$ /ERROR/stderr:
$ /ERROR/stderr: File "/Users/jeffhall/.virtualenvs/virtenv/lib/python3.6/site-packages/locust/runners.py", line 465, in client_listener
    events.slave_report.fire(client_id=msg.node_id, data=msg.data)
$ /ERROR/stderr:
$ /ERROR/stderr: File "/Users/jeffhall/.virtualenvs/virtenv/lib/python3.6/site-packages/locust/events.py", line 34, in fire
    handler(**kwargs)
$ /ERROR/stderr:
$ /ERROR/stderr: File "/Users/jeffhall/.virtualenvs/virtenv/lib/python3.6/site-packages/locust/stats.py", line 674, in on_slave_report
    global_stats.errors[error_key] = StatsError.from_dict(error)
$ /ERROR/stderr:
$ /ERROR/stderr: File "/Users/jeffhall/.virtualenvs/virtenv/lib/python3.6/site-packages/locust/stats.py", line 627, in from_dict
    data["occurrences"]
$ /ERROR/stderr:
$ /ERROR/stderr: KeyError: 'occurrences'
$ /ERROR/stderr:
$ /ERROR/stderr: 2021-10-21T15:52:47Z
$ /ERROR/stderr:
$ /ERROR/stderr: <Greenlet at 0x10990b648: <bound method MasterLocustRunner.client_listener of <locust.runners.MasterLocustRunner object at 0x109983438>>> failed with KeyError

I then found this comment about a typo https://github.com/anhldbk/swarm/blob/master/src/main/java/com/bigsonata/swarm/common/stats/StatsError.java#L30 and noticed that looks like the typo has been fixed... at least in locust 0.14.6 which is what I'm running and it looks like its been fixed since 0.12.0! (See: locustio/locust#1024 & locustio/locust@f0a5f89 )

I forked and fixed up the word on the Swarm side and it seemed to help me get around my error! Figured I could offer this fix here too!

[anhldbk]

@jeffhall-wk Change looks great. Thank you!