FTAG: fix vertex labelling bug

This fixes a bug in the ftag truth vertex labelling spotted by @npond.

Reference: (before !62888 (merged) and !62544 (merged))

values: [-2 -1  0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15]
counts: [  473 13495  1676    84   105   117   107   113    99    77    52    33
    28    13     4     2     1     1]

Current:

values: [-2 -1  0  1  8]
counts: [  473 13495  2509     2     1]

This MR:

values: [-2 -1  0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21
 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 38 43]
counts: [  473 13495  1676    38    29    35    30    26    30    25    37    35
    27    33    38    41    38    38    29    19    46    37    30    29
    30    12    19    13     5     6    20     8     2     4     1     4
     8     1     3     8     2]

Note that the new labelling is per-event rather than per-jet so the vertex indices are higher in the current scheme. What is important is that the number of vertex indices == 0 (PV) is now equal (1676) and the number vertex indices >0 is also equal.

cc @jmwagner @jburzyns