fix: Update Javadoc reference to renamed mapMutable method

Update @see reference and example code in SignalModifier to use the
renamed mapMutable method instead of the old map method name.

https://claude.ai/code/session_019LonZZA33kKDmk7jCgN7cT

Author: claude

signals/src/main/java/com/vaadin/signals/function/SignalModifier.java

@@ -38,8 +38,8 @@
  *
  * ValueSignal<Todo> todoSignal = new ValueSignal<>(
  *         new Todo("Buy milk", false));
- * WritableSignal<Boolean> doneSignal = todoSignal.map(Todo::isDone,
- *         Todo::setDone);
+ * WritableSignal<Boolean> doneSignal = todoSignal.mapMutable(
+ *         Todo::isDone, Todo::setDone);
  *
  * doneSignal.value(true); // Calls todoSignal.modify(t -> t.setDone(true))
  * </pre>
@@ -48,7 +48,7 @@
  *            the parent signal value type
  * @param <C>
  *            the child (mapped) signal value type
- * @see ValueSignal#map(SignalMapper, SignalModifier)
+ * @see ValueSignal#mapMutable(SignalMapper, SignalModifier)
  */
 @FunctionalInterface
 public interface SignalModifier<P, C> {