Update mappings.vim

Use noremap instead of map to prevent recursive mappings.
you shouldn't make these kind of mappings (where rhs is `<Plug>something`) no recursive because `<Plug>something` is meant to be expanded to another mapping.
it's mapped here: https://github.com/LaTeX-Box-Team/LaTeX-Box/blob/master/ftplugin/latex-box/motion.vim#L249