LibWeb: Use correct offset value when replacing character data

Previously, the range's end offset was being set using it's previous
value.

Tests/LibWeb/Text/input/DOM/Delete-from-text-node-with-associated-range.html

@@ -0,0 +1,15 @@
+<script src="../include.js"></script>
+<script>
+    test(() => {
+        const text = new Text("foobar");
+
+        const range = document.createRange();
+        range.setEnd(text, 5);
+        
+        text.deleteData(5, 1);
+        text.deleteData(4, 1);
+        text.deleteData(3, 1);
+
+        println(text.data);
+    })
+</script>