scripts/spelling.txt: remove 'thead' as a typo

scripts/spelling.txt: remove 'thead' as a typo

T-Head is a vendor of processor core IP, and they have recently introduced
the RISC-V TH1520 SoC.  Remove 'thead' as a typo of 'thread' to avoid
checkpatch incorrectly warning that 'thead' is typo in patches that add
support for T-Head designs in the kernel.

Link: https://lkml.kernel.org/r/20230723010329.674186-1-dfustini@baylibre.com
Link: https://www.t-head.cn/
Signed-off-by: Drew Fustini <dfustini@baylibre.com>
Acked-by: Guo Ren <guoren@kernel.org>
Cc: Conor Dooley <conor@kernel.org>
Cc: Jisheng Zhang <jszhang@kernel.org>
Cc: Colin Ian King <colin.i.king@gmail.com>
Cc: Diederik de Haas <didi.debian@cknow.org>
Cc: Ian Rogers <irogers@google.com>
Cc: Luca Ceresoli <luca.ceresoli@bootlin.com> # versaclock5
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>