[share_plus] Format bug in CHANGELOG.md

[HakkyuKim]

See 2.1.2 and 2.1.3 in https://pub.dev/packages/share_plus/changelog#300.

[Boehrsi]

Thanks for the hint, I will create a PR tomorrow.

[thecoderkd]

Hey @Boehrsi, I sent in the PR, since it was a minor issue. I hope you don't mind.

[Boehrsi]

Sure no problem go ahead :).