scripts/spelling.txt: add "oveflow"

Add typo "oveflow" for "overflow".  This typo was found and fixed in
tools/testing/selftests/bpf/prog_tests/btf_dump.c

Link: https://lore.kernel.org/all/20211122070528.837806-1-dfustini@baylibre.com/
Link: https://lkml.kernel.org/r/20211122072302.839102-1-dfustini@baylibre.com
Signed-off-by: Drew Fustini <dfustini@baylibre.com>
Suggested-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Cc: Colin Ian King <colin.king@intel.com>
Cc: Drew Fustini <dfustini@baylibre.com>
Cc: zuoqilin <zuoqilin@yulong.com>
Cc: Tom Saeger <tom.saeger@oracle.com>
Cc: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>