Fix duplicate MBTA EarlyWarning citation

We've seen a superevent with multiple references to the same EarlyWarning pipeline MBTA:

The candidate was found by the GstLAL [1], GstLAL SSM [2], MBTA [3], MBTA SSM [4], PyCBC Live [5], and SPIIR [6] analysis pipelines. An early-warning alert was issued for this candidate, detected by the GstLAL [7], MBTA [3], MBTA [3], and PyCBC Live [8] early-warning pipelines.

with citations:

 [2] Hanna et al. (2024) arXiv 2412.10951
 [3] Allene et al. CQG 42, 105009 (2025) doi:10.1088/1361-6382/add234
 [4] Allene et al. CQG 42, 105009 (2025) doi:10.1088/1361-6382/add234
 [5] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a

From this superevent: https://gracedb-playground.ligo.org/superevents/S250605bd/view/