Mising `:` after the key of sign-off trailer

[semioticrobotic]

I'm delighted to have sign-off functionality in gitui!

I notice that the trailer does not include the requisite : character required for properly formatting the trailer. I need to add this character every time, which can be tedious.

I see this omission noted in comments on a prior commit:

There's the : missing after the key of the trailer.

Originally posted by @matthiasbeyer in #1758 (comment)

Is resolving this issue still possible?

[extrawurst]

sure please open a PR