fix feature typo

[ihciah]

No description provided.

[dhardy]

LGTM.

Unrelated error:

error[E0658]: use of unstable library feature 'trusted_len'
   --> src/distributions/distribution.rs:163:15
    |
163 | impl<D, R, T> iter::TrustedLen for DistIter<D, R, T>
    |               ^^^^^^^^^^^^^^^^
    |
    = note: see issue #37572 <https://github.com/rust-lang/rust/issues/37572> for more information
    = help: add `#![feature(trusted_len)]` to the crate attributes to enable

error[E0200]: the trait `TrustedLen` requires an `unsafe impl` declaration
   --> src/distributions/distribution.rs:163:1
    |
163 | impl<D, R, T> iter::TrustedLen for DistIter<D, R, T>
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: the trait `TrustedLen` enforces invariants that the compiler can't check. Review the trait documentation and make sure this implementation upholds those invariants before adding the `unsafe` keyword
help: add `unsafe` to this trait implementation
    |
163 | unsafe impl<D, R, T> iter::TrustedLen for DistIter<D, R, T>
    | ++++++