Unreleased version link should use lower case

[jasongerbes]

Hi there, just a minor issue I noticed on the Keep a Changelog website.

Currently the [unreleased] link in the Version 1.0.0 example uses title case:

[Unreleased]: https://github.com/olivierlacan/keep-a-changelog/compare/v1.0.0...HEAD

This should be lowercase (i.e. [unreleased]:...) as reference links are case-insensitive, and the current example conflicts with markdown formatters (e.g. Prettier).

Also worth noting that this repo's CHANGELOG.md also uses lowercase.

[olivierlacan]

I'm a bit curious why Prettier would have an opinion here. This is an editorial choice, not something up to a linter.

[unreleased] would display lowercase heading, which is not the intent, even if that matches more clearly, Markdown reference links are not case-sensitive, so the reference link works fine.