fix(Changelog.md): typo
- `Port<T> = PortGeneric<T, ReadWriteAccess>`
- `PortReadOnly<T> = PortGeneric<T, ReadOnlyAccess>`
- `PortWriteOnly<T> = PortGeneric<T, WriteOnlyAccess>`
-- The following methods no longer require the `nightly` feature to be `const fn`s` ([#255](https://github.com/rust-osdev/x86_64/pull/255)):
+- The following methods no longer require the `nightly` feature to be `const fn`s ([#255](https://github.com/rust-osdev/x86_64/pull/255)):
- `PageTable::new`
- `GlobalDescriptorTable::from_raw_slice`
- `MappedFrame::{start_address, size}`
