Resolved warnings about unused result in concurrency.md

[thyrgle]

The example code in the Channels subsection of the rust book give warnings about

unused result which must be used, #[warn(unused_must_use)] on by default

Added a small pattern match to resolve those warnings.

[rust-highfive]

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[alexcrichton]

Perhaps these could be changed to .unwrap()? I think it indicates a program bug if Err is returned.

[thyrgle]

Thanks. Sorry about that. Uses .unwrap() instead of empty variable.

[alexcrichton]

@bors: r+ bfb7361 rollu

Thanks!

[alexcrichton]

@bors: rollup

[bors]

⌛ Testing commit bfb7361 with merge 078aff8...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-opt