lib/Kconfig.debug: fix grammar in RUST_BUILD_ASSERT_ALLOW

Just a grammar fix in lib/Kconfig.debug, under the config option
RUST_BUILD_ASSERT_ALLOW.

Reported-by: Miguel Ojeda <ojeda@kernel.org>
Closes: https://github.com/Rust-for-Linux/linux/issues/1006


Fixes: ecaa6ddf ("rust: add `build_error` crate")
Signed-off-by: Timo Grautstueck <timo.grautstueck@web.de>
Link: https://lore.kernel.org/r/20241006140244.5509-1-timo.grautstueck@web.de


Signed-off-by: Miguel Ojeda <ojeda@kernel.org>