mail/claws-mail-plugins: improve pkg-descr

Since the original and maintainer-approved prior pkg-descr was obviously
wrong, let's play this pragmatically:

I am taking the liberty to commit this improvement,
but am awaiting for the maintainer's final word
on whether we should use this, or a different pkg-descr.

PR: 251319 (pkg-descr fix)
PR: 245939 (initial proposal to introduce mismatched pkg-descr)
Submitted by: bsd@dino.sk
Reported by: FreeBSD@ShaneWare.Biz