Add custom finishing function example to the "Fallible Builders" chap… #927

Job	Run time
version-sync	3s
cargo-lock	10s
cargo-machete	6s
cargo-doc (ubuntu)	24s
cargo-fmt	17s
cargo-miri (--locked)	1m 58s
cargo-miri	1m 49s
compilation-benchmarks	2m 4s
cargo-doc (windows)	2m 22s
runtime-benchmarks (args_3)	58s
cargo-doc (macos)	31s
test-msrv (ubuntu)	4m 44s
test-stable (ubuntu, --locked)	1m 17s
runtime-benchmarks (args_5)	7m 21s
test-msrv (windows)	5m 51s
runtime-benchmarks (args_10_alloc)	53s
test-stable (ubuntu)	1m 17s
test-msrv (macos)	3m 20s
test-unstable (beta, --locked)	28s
test-stable (windows, --locked)	4m 11s
runtime-benchmarks (args_10_structs)	1m 19s
test-unstable (beta)	30s
runtime-benchmarks (args_10)	2m 11s
test-stable (windows)	4m 37s
test-unstable (nightly, --locked)	26s
test-stable (macos, --locked)	1m 24s
runtime-benchmarks (args_20)	1m 32s
test-stable (macos)	1m 16s
test-unstable (nightly)	26s
prettier	9s
taplo-fmt	4s
typos	4s
website-build	21s
	54m 23s

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