Fix Dart 3.6.0 linter issues.

Author: renggli

lib/src/xml/mixins/has_writer.dart

@@ -33,7 +33,7 @@ mixin XmlHasWriter implements XmlHasVisitor {
   /// - If the [sortAttributes] is provided, attributes are on-the-fly sorted
   ///   using the provided [Comparator].
   /// - If the predicate [spaceBeforeSelfClose] returns `true`, self-closing
-  ///   elements will be closed with a space before the slash ('<example />')
+  ///   elements will be closed with a space before the slash (`<example />`).
   ///
   String toXmlString({
     bool pretty = false,