Fix off-by-one in FACTOR_FINDER

The first batch number passed to the first thread of FACTOR_FINDER method was off-by-one. (In practice, this makes little to no difference, but there's no reason to allow the edge case to stand.)

Full Changelog: v6.2.3...v6.2.4

sha1sum results:
c610448b9f449e4b13483b169328c7fe26f26efe FindAFactor-6.2.4-cp310-cp310-manylinux_2_35_x86_64.whl
869ccfa97ea32e3358f6bb6d5cebc0b33ef985b3 FindAFactor-6.2.4-cp312-cp312-manylinux_2_39_x86_64.whl
09c7b676c5951787bdf568b09c1865bd18b14852 FindAFactor-6.2.4-cp312-cp312-win_amd64.whl
dac597e0e177f7757865bc98246d964a968b5339 FindAFactor-6.2.4-cp313-cp313-macosx_13_0_x86_64.whl
a66085fd5c8a8512821e9bbe31c9c06ed5ca0f70 FindAFactor-6.2.4-cp313-cp313-macosx_14_0_arm64.whl
965b011c44ee4ae8677e8330dd76e1303f29472e FindAFactor-6.2.4-cp313-cp313-macosx_15_0_arm64.whl
37235ba2890686f717821721a2fbde6bcd79eefc FindAFactor-6.2.4-cp38-cp38-manylinux_2_31_x86_64.whl
cf209487e544fbb67ae0db5c5b50fff32a50b09b findafactor-6.2.4.tar.gz