[strings] Fix printf formatting to be less like credentials

git secrets keeps complaining that the previous string was a
possible credential. This patch changes it to be less like a
credential and removes the annoying warning.

Change-Id: I5074a4e3c11ab0d689b1a88e8d3eec0794dad899
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3070699Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Sathya Gunasekaran  <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#76081}

src/strings/string-stream.cc

@@ -365,7 +365,7 @@ void StringStream::PrintMentionedObjectCache(Isolate* isolate) {
   if (object_print_mode_ == kPrintObjectConcise) return;
   DebugObjectCache* debug_object_cache =
       isolate->string_stream_debug_object_cache();
-  Add("==== Key         ============================================\n\n");
+  Add("-- ObjectCacheKey --\n\n");
   for (size_t i = 0; i < debug_object_cache->size(); i++) {
     HeapObject printee = *(*debug_object_cache)[i];
     Add(" #%d# %p: ", static_cast<int>(i),