[vm] Add whitespace before parenthesis.

Follow-up to https://dart-review.googlesource.com/c/sdk/+/97560.

Change-Id: I83cc296238f33ec1c58994393c1b416976ff383a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/97564
Reviewed-by: Alexander Aprelev <aam@google.com>
diff --git a/runtime/vm/object_graph.cc b/runtime/vm/object_graph.cc
index 20ee679..71f74b6 100644
--- a/runtime/vm/object_graph.cc
+++ b/runtime/vm/object_graph.cc
@@ -39,7 +39,7 @@
 
   // Marks and pushes. Used to initialize this stack with roots.
   // We can use ObjectIdTable normally used by serializers because it
-  // won't be in use while handling a service request(only use for ObjectGraph).
+  // won't be in use while handling a service request (ObjectGraph's only use).
   virtual void VisitPointers(RawObject** first, RawObject** last) {
     Heap* heap = isolate()->heap();
     for (RawObject** current = first; current <= last; ++current) {