Fix deprecated vim.validate table syntax

[ChausseBenjamin]

This PR fixes the deprecation warning for vim.validate by replacing the deprecated table syntax with the new positional arguments syntax.

The warning from :checkhealth was:

- ⚠️ WARNING vim.validate{<table>} is deprecated. Feature will be removed in Nvim 1.0
  - ADVICE:
    - use vim.validate(<params>) instead.
    - stack traceback:
        ~/.local/share/nvim/site/pack/core/opt/yadm.nvim/lua/yadm/init.lua:6
        ~/.local/share/nvim/site/pack/core/opt/yadm.nvim/lua/yadm/init.lua:50
        ~/.config/nvim/lua/plugins/fugitive.lua:16
        [C]:-1
        ~/.config/nvim/lua/bundles/git.lua:13
        [C]:-1
        ~/.config/nvim/init.lua:48
    - stack traceback:
        ~/.local/share/nvim/site/pack/core/opt/yadm.nvim/lua/yadm/init.lua:9
        ~/.local/share/nvim/site/pack/core/opt/yadm.nvim/lua/yadm/init.lua:50
        ~/.config/nvim/lua/plugins/fugitive.lua:16
        [C]:-1
        ~/.config/nvim/lua/bundles/git.lua:13
        [C]:-1
        ~/.config/nvim/init.lua:48

Changes:

• Changed vim.validate({ config = { config, "table", true } }) to vim.validate("config", config, "table", true)
• Changed vim.validate({ yadm_dir = { config.yadm_dir, "string" } }) to vim.validate("yadm_dir", config.yadm_dir, "string")