docs(mason): more generic config snippet (#550)

doc/mason.txt

@@ -14,11 +14,15 @@ the `server.cmd` setting (see |rustaceanvim.config| and |RustaceanLspClientOpts|
     server = {
       cmd = function()
 	local mason_registry = require('mason-registry')
-	local ra_binary = mason_registry.is_installed('rust-analyzer') 
-	  -- This may need to be tweaked, depending on the operating system.
-	  and mason_registry.get_package('rust-analyzer'):get_install_path() .. "/rust-analyzer"
-	  or "rust-analyzer"
-	return { ra_binary } -- You can add args to the list, such as '--log-file'
+	if mason_registry.is_installed('rust-analyzer') then
+	  -- This may need to be tweaked depending on the operating system.
+	  local ra = mason_registry.get_package('rust-analyzer')
+	  local ra_filename = ra:get_receipt():get().links.bin['rust-analyzer']
+	  return { ('%s/%s'):format(ra:get_install_path(), ra_filename or 'rust-analyzer') }
+	else
+	  -- global installation
+	  return { 'rust-analyzer' }
+	end
       end,
     },
   }