[heap] Fix typo: the flag is called '--track-retaining-path'

R=ulan@chromium.org

Change-Id: Ib35e3556f11b1c7beb6aa908ac72d0a1bd9d8639
Reviewed-on: https://chromium-review.googlesource.com/958923Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51885}

src/heap/heap.cc

@@ -440,7 +440,7 @@ void Heap::ReportStatisticsAfterGC() {
 void Heap::AddRetainingPathTarget(Handle<HeapObject> object,
                                   RetainingPathOption option) {
   if (!FLAG_track_retaining_path) {
-    PrintF("Retaining path tracking requires --trace-retaining-path\n");
+    PrintF("Retaining path tracking requires --track-retaining-path\n");
   } else {
     int index = 0;
     Handle<FixedArrayOfWeakCells> array = FixedArrayOfWeakCells::Add(