fix: duplicated words in signals-unix.c and desugaring.jl comments (#61802)

Two one-line typo fixes for duplicated words in source comments:
- `src/signals-unix.c` — "redefine this as as an \"unreachable reached\"
error message" → "redefine this as an \"unreachable reached\" error
message"
- `JuliaLowering/src/desugaring.jl` — "is the list of statements which
needs to to be emitted" → "...which needs to be emitted"

No code/behavior change.

Co-authored-by: Maya Chen <275405107+otjdiepluong@users.noreply.github.com>