Fixed name for dependency "textobj-user"

Hi,

I fixed the name of the "textobj-user" dependency in addon-info.json.

For VAM, the plugin names at vim.org are canonical and there the plugin is called "textobj-user". This is in fact the official name: try `:h textobj-user`. The "vim-" is just a prefix people on Github use to mark their Vim plugin repos.

For consistency you may want to change this line in the same fashion, but that's up to you:

    "name" : "vim-textobj-latex"