Fix broken POSIX basedefs links for nested headers in llvm-libc docs. The docgen script currently emits paths like `sys/wait.h.html`, but the Open Group uses `sys_wait.h.html`, for example: - https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/sys_wait.h.html This updates nested-header link generation while leaving flat headers unchanged.