fix changelog v1.2.1 style

[xuexb]

before: https://github.com/royriojas/flat-cache/blob/master/changelog.md#v121

after: https://github.com/xuexb/flat-cache/blob/master/changelog.md#v121

Thanks~

[royriojas]

hi @xuexb, thank you for your PR. Will be merged. But please note that the changelog is automatically created by changelogx which means you had found a bug in the log generation. Will make the fix also on the changelogx module.

Thank you again for finding and fixing the issue.