fix(Changelog.md): typo (#278)

Author: toku-sa-n

Changelog.md

@@ -6,7 +6,7 @@
   - `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}`