numfmt: fix Unit::SI uses lowercase `k` suffix for kilos #6387

Job	Run time
Build the fuzzers	2m 3s
Run the fuzzers (fuzz_test, true)	2m 23s
Run the fuzzers (fuzz_date, false)	1m 31s
Run the fuzzers (fuzz_expr, true)	2m 30s
Run the fuzzers (fuzz_printf, false)	2m 27s
Run the fuzzers (fuzz_echo, true)	2m 32s
Run the fuzzers (fuzz_seq, false)	1m 44s
Run the fuzzers (fuzz_sort, false)	1m 46s
Run the fuzzers (fuzz_wc, false)	1m 33s
Run the fuzzers (fuzz_cut, false)	1m 35s
Run the fuzzers (fuzz_split, false)	1m 55s
Run the fuzzers (fuzz_tr, false)	1m 20s
Run the fuzzers (fuzz_env, false)	1m 34s
Run the fuzzers (fuzz_cksum, false)	1m 28s
Run the fuzzers (fuzz_parse_glob, true)	2m 14s
Run the fuzzers (fuzz_parse_size, true)	1m 13s
Run the fuzzers (fuzz_parse_time, true)	2m 20s
Run the fuzzers (fuzz_seq_parse_number, true)	2m 17s
	34m 25s

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