doc test: fix mpsc.rs try_send doc test

[eval-exec]

This Pr want to fix the doctest, to make #145293 's CI pass:

r? @Zalathar

https://github.com/rust-lang/rust/actions/runs/16903356990/job/47887354221

2025-08-12T10:19:32.3873237Z test library/std/src/thread/scoped.rs - thread::scoped::ScopedJoinHandle<'scope,T>::join (line 302) ... ok
2025-08-12T10:19:32.4370250Z test library/std/src/time.rs - time::SystemTimeError::duration (line 688) ... ok
2025-08-12T10:19:32.5121966Z test library/std/src/time.rs - time::UNIX_EPOCH (line 664) ... ok
2025-08-12T10:19:32.5122586Z 
2025-08-12T10:19:32.5122738Z failures:
2025-08-12T10:19:32.5122973Z 
2025-08-12T10:19:32.5123482Z ---- library/std/src/sync/mpsc.rs - sync::mpsc::SyncSender<T>::try_send (line 691) stdout ----
2025-08-12T10:19:32.5124286Z Test executable failed (exit status: 1).
2025-08-12T10:19:32.5124518Z 
2025-08-12T10:19:32.5124605Z stdout:
2025-08-12T10:19:32.5124810Z message 3 received
2025-08-12T10:19:32.5125043Z message 1 received
2025-08-12T10:19:32.5125288Z the third message was never sent
2025-08-12T10:19:32.5125497Z 
2025-08-12T10:19:32.5125581Z stderr:
2025-08-12T10:19:32.5125701Z 
2025-08-12T10:19:32.5125935Z thread '<unnamed>' (203874) panicked at library/std/src/sync/mpsc.rs:14:25:
2025-08-12T10:19:32.5126459Z called `Result::unwrap()` on an `Err` value: SendError { .. }
2025-08-12T10:19:32.5126836Z stack backtrace:
2025-08-12T10:19:32.5127568Z ␛[0m␛[1m␛[38;5;9merror␛[0m␛[0m␛[1m: the main thread terminated without waiting for all remaining threads␛[0m
2025-08-12T10:19:32.5127971Z 
2025-08-12T10:19:32.5128335Z ␛[0m␛[1m␛[38;5;10mnote␛[0m␛[0m␛[1m: set `MIRIFLAGS=-Zmiri-ignore-leaks` to disable this check␛[0m
2025-08-12T10:19:32.5128694Z 
2025-08-12T10:19:32.5128943Z ␛[0m␛[1m␛[38;5;9merror␛[0m␛[0m␛[1m: aborting due to 1 previous error␛[0m
2025-08-12T10:19:32.5129519Z 
2025-08-12T10:19:32.5129527Z 
2025-08-12T10:19:32.5129532Z 
2025-08-12T10:19:32.5129537Z 
2025-08-12T10:19:32.5129631Z failures:
2025-08-12T10:19:32.5130018Z     library/std/src/sync/mpsc.rs - sync::mpsc::SyncSender<T>::try_send (line 691)
2025-08-12T10:19:32.5130396Z 
2025-08-12T10:19:32.5130713Z test result: FAILED. 999 passed; 1 failed; 16 ignored; 0 measured; 344 filtered out; finished in 105.92s

[rustbot]

r? @ibraheemdev

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[Zalathar]

I can't approve this because I'm not on the appropriate team, but it does seem simpler and more reliable than the existing test.

r? libs