correct typo on locust datapoint

[thomasdegiacinto]

At its beginning Locust had a typo in metrics return during test. Since locustio/locust@f0a5f89 the typo has been fixed.

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• Description of PR explains the context of change
• Unit tests cover the change, no broken tests
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• Documentation update ('available in the unstable snapshot' warning if necessary)
• Changes file inside site/dat/docs/changes directory, one-line note of change inside