Changelog script: minor complaints

[aaruni96]

Originally posted by @fingolfin at oscar-system/Oscar.jl#5544 (comment) . Posting here, just so its easy to find for me.

I discussed this with @aaruni96 and he'll improve the script: instead of inserting "garbage", it will insert a suitable message about a parsing problem or so

I would suggest that we also make the script handle lists formatted with * instead of -.