Fix inverted S/C relevance position mapping in prepare_trec_eval_files.py

[iamjaygao]

prepare_trec_eval_files.py maps S to position 2 and C to position 3.
With -m 'ndcg.1=0,2=0.01,3=0.1,4=1' that gives S a gain of 0.01 and C
a gain of 0.1 — reversed. train.py uses S=0.1, C=0.01.

Correct mapping per @franbvalero in #9: E=4, S=3, C=2, I=1.

Also reported in #18. The trailing-newline change is from the web editor.

[dst111dst]

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[iamjaygao]

Added a minimal reproduction — no dataset, no dependencies:
https://github.com/iamjaygao/esci-gain-mapping-repro