Fix channel example in async blog post

[mre]

Fixes the channel example in the async blog post so it actually sends file contents through the channel.

• Each spawned task now sends contents via tx with a proper Result<(), Box<dyn Error + Send + Sync>> return type so ? works on both fs::read_to_string and tx.send
• Clones tx per task and drops the original after the scope so the rx loop terminates
• Removes unused imports
• Updates playground link
• Bumps updated date