[docs] Fix formatting for merge_tdist function

[MasterOdin]

Description

PR fixes the formatting for the merge_tdist function so that it'll appear on its own line, as right now it appears under the construct_tdigest function which isn't correct.

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[steveburnett]

LGTM! (docs)

Pull branch, local doc build, review local doc build compared to published doc.

Thanks for catching this!