Consider removing `v` denotation from GitHub compare URLs

[Archmonger]

The v in GitHub tags such as v0.0.1 and v0.0.2 in https://github.com/olivierlacan/keep-a-changelog/compare/v0.0.1...v0.0.2 is zero-value added denotator.

To encourage easier syntax validation and reduce a denotation that is easy to accidentally forget, I personally believe it shouldn't be encouraged in the primary Keep a Changelog example.

Additionally, it makes it a bit easier for me to maintain my new Keep a Changelog validation script.