ed25519: Docs mention feature "std" which is not defined

[lorenzleutgeb]

signatures/ed25519/src/lib.rs

Lines 238 to 239 in f2b087c

	//! - `std` *(default)*: Enable `std` support in [`signature`], which currently only affects whether
	//! [`signature::Error`] implements `std::error::Error`.

is in conflict with

signatures/ed25519/Cargo.toml

Lines 34 to 38 in f2b087c

	[features]
	default = ["alloc"]
	alloc = ["pkcs8?/alloc", "signature/alloc"]
	pem = ["alloc", "pkcs8/pem"]
	serde = ["dep:serdect"]