Typo for analysis basis in Jetstream config docs

[mikewilli]

exposure_basis should be analysis_bases on this line https://github.com/mozilla/experimenter-docs/blob/2df4fc46607ffa9bbd614401fbef91c1d4f362bd/docs/jetstream/configuration.md?plain=1#L205, like so:

analysis_bases = ["exposures", "enrollments"]

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