Update PR template changelog checklist
Update PR template changelog checklist

Update the language to reflect that entries in the changelog should
only be end user focused changes. Motived by @tsibley's comment¹ in a
recent PR.

It's nice for external users if our changelog is not bloated by
internal development changes!

¹ https://github.com/nextstrain/augur/pull/1039#discussion_r1014473787

nextstrain/augur,nextstrain/augur,nextstrain/augur