nixos/*: use properly indented strings for option docs

using regular strings works well for docbook because docbook is not as
whitespace-sensitive as markdown. markdown would render all of these as
code blocks when given the chance.

nixos/modules/services/databases/mysql.nix

@@ -36,9 +36,9 @@ in
       package = mkOption {
         type = types.package;
         example = literalExpression "pkgs.mariadb";
-        description = "
+        description = ''
           Which MySQL derivation to use. MariaDB packages are supported too.
-        ";
+        '';
       };
 
       user = mkOption {