Correct the code for publishing main branch snapshots

[keynmol]

Haven't used this plugin before, but I believe that my reading comprehension skills are not fully broken and this is indeed a typo

[eed3si9n]

Thanks @keynmol!

[mzuehlke]

When reading the README.md I couldn't make sense out of the sentence

Note the use of `+=` rather than `:=`.

which after this change makes no sense anymore and should be removed.

Btw, as the default value is:

githubWorkflowPublishTargetBranches := Seq(RefPredicate.Equals(Ref.Branch("main")))

I don't see a typo here.

Got introduce here: 612e85d