Fix wording on AssertLayoutTreeUpdated comments

Address the review comment of
https://chromium-review.googlesource.com/c/chromium/src/+/920329/7#message-b9399a526b29ba08927301f8baec6c8d30c20682

TBR=futhark@chromium.org

Bug: 776656
Change-Id: I3e6084b7dde05df54fddf38bfeb3220c9c585bb2
Reviewed-on: https://chromium-review.googlesource.com/923667
Commit-Queue: Hayato Ito <hayato@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Reviewed-by: Hayato Ito <hayato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537614}

third_party/WebKit/Source/core/dom/Document.cpp

@@ -2077,7 +2077,7 @@ static void AssertLayoutTreeUpdated(Node& root) {
     DCHECK(!node.NeedsStyleInvalidation());
     DCHECK(!node.ChildNeedsStyleInvalidation());
     // Make sure there is no node which has a LayoutObject, but doesn't have a
-    // parent in a flat tree. If there is such a node, we forgot to dettach a
+    // parent in a flat tree. If there is such a node, we forgot to detach the
     // node. DocumentNode is only an exception.
     DCHECK((node.IsDocumentNode() || !node.GetLayoutObject() ||
             FlatTreeTraversal::Parent(node)))