Fix typo (it's/its) in join_all docs #2207

Job	Run time
cargo bench	1m 20s
codegen	33s
cargo clippy	43s
cargo +stable build	1m 1s
cargo +beta build	1m 24s
cargo +nightly build	1m 11s
cargo +1.36 build (futures-{core, io, sink})	1m 34s
cargo test --target aarch64-unknown-linux-gnu	9m 20s
cargo test --target armv7-unknown-linux-gnueabihf	10m 40s
cargo test --target i686-unknown-linux-gnu	5m 25s
cargo doc	34s
cargo hack check --feature-powerset	5m 28s
cargo fmt	10s
cargo minimal-versions build	43s
cargo miri test	8m 59s
cargo build --target thumbv7m-none-eabi	40s
cargo build --target thumbv6m-none-eabi	44s
cargo test -Z sanitizer=address	5m 54s
cargo test -Z sanitizer=memory	3m 16s
cargo test -Z sanitizer=thread	3m 15s
cargo test (ubuntu-latest)	4m 19s
cargo test (macos-latest)	7m 11s
cargo test (windows-latest)	6m 25s
cargo +1.56 build	2m 4s
	1h 22m 53s

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