bip-0324: fix git instruction order in test_sage_decoding.py

[theStack]

Tiny fix correcting the order of commands, for git checkout one has to change into the repository directory first.

[stratospher]

(unrelated to this PR but related to BIP 324) - you could also remove https://github.com/bitcoin/bips/blob/master/bip-0324/xswiftec_test_vectors.csv maybe? xswiftec_test_vectors.csv isn't being used anywhere and I'd assume it's a leftover from the previous revision of the doc which replaced it with xswiftec_inv_test_vectors.csv(currently used).

[luke-jr]

@real-or-random @jonasschnelli @sipa

[sipa]

LGTM