Normalize install_dir in health check

When passing an empty string to config.get_install_dir() then
config.install_dir path will get concatinated with an empty string which
adds a trailing slash. This will cause that the path won't be recognized
as being in runtimepaths even though it actually is.

lua/nvim-treesitter/health.lua

@@ -87,7 +87,7 @@ local function install_health()
     health.info(k .. ': ' .. v)
   end
 
-  local installdir = config.get_install_dir('')
+  local installdir = vim.fs.normalize(config.get_install_dir(''))
   health.start('Install directory for parsers and queries')
   health.info(installdir)
   if vim.uv.fs_access(installdir, 'w') then