Fix source comments and user-facing messages

[nirmal-mina]

Summary

Fixes small but user-facing text/comment issues in source files.

Changes

• 09/threads-safe.c: change usage string from usage: threads <loops> to usage: threads-safe <loops>.
• 08/pipe.c: update comment from “redirect standard output to a file” to “redirect standard output to a pipe”.
• scripts/notify.py: improve wording in comments/help text.

Testing

• Verified text-only changes do not affect behavior.