use proper order of arguments for gumbo_vector_add
use proper order of arguments for gumbo_vector_add

Sigil-Ebook/sigil-gumbo,Sigil-Ebook/sigil-gumbo,Sigil-Ebook/sigil-gumbo,Sigil-Ebook/sigil-gumbo,Sigil-Ebook/sigil-gumbo