Fix incorrect feature flag documented on join! macro

[RAnders00]

The join! and try_join! macros are behind the async-await-macro feature flag, not async-await.

[taiki-e]

Hmm. The current one is correct for the futures documentation, but indeed incorrect for the futures-util documentation.

futures-rs/futures-util/Cargo.toml

Lines 17 to 18 in ae3297f

	async-await = []
	async-await-macro = ["async-await", "futures-macro"]

futures-rs/futures/Cargo.toml

Line 39 in ae3297f

async-await = ["futures-util/async-await", "futures-util/async-await-macro"]

0.4 should fix the difference here (#2308 (comment), #2295), but it is not clear how we should handle this in 0.3.

[taiki-e]

Closing -- as said above, 0.3 requires a more complex fix and 0.4 does not need to do anything. In any case, this PR will not work as a corrective to any of these as it is.

Thanks anyway for the PR!