Fix typo in comment within nsFrame.cpp (no bug) rs=dbaron DONTBUILD

layout/generic/nsFrame.cpp

@@ -6629,7 +6629,7 @@ nsFrame::GetParentStyleContextFrame(nsPresContext* aPresContext,
  * is needed because the split inline's style context is the parent of the
  * anonymous block's style context.
  *
- * If aFrame is not ananonymous block, null is returned.
+ * If aFrame is not an anonymous block, null is returned.
  */
 static nsIFrame*
 GetIBSpecialSiblingForAnonymousBlock(nsIFrame* aFrame)