bootstrap: fix broken INITNSMGR-CONCURRENCY-DESIGN.md reference

After commit 589a1044e6 (parent repo) moved
local/docs/INITNSMGR-CONCURRENCY-DESIGN.md to
local/docs/legacy-obsolete-2026-07-25/, the doc reference in
bootstrap/src/initnsmgr.rs became broken. Update it to point at
the archive path so future readers can find the design rationale.

Companion to the LG Gram Round 1 parent-repo commit (same date).

bootstrap/src/initnsmgr.rs

@@ -15,7 +15,7 @@ use redox_rt::proc::FdGuard;
 // single-threaded Rc<RefCell>. NOTE: this Mutex is a spinlock -- never hold it
 // across a blocking syscall; resolve the cap_fd under the lock, clone the Arc,
 // and do the blocking openat with the lock released. See
-// local/docs/INITNSMGR-CONCURRENCY-DESIGN.md.
+// local/docs/legacy-obsolete-2026-07-25/INITNSMGR-CONCURRENCY-DESIGN.md.
 use redox_rt::sync::Mutex;
 use redox_scheme::{
     CallerCtx, Id, IntoTag, OpenResult, RequestKind, Response, SendFdRequest, SignalBehavior,