Mention `ClassLinker::LinkFieldsHelper::LinkFields` in ART's doc.

Replace mentions of `ClassLinker::LinkFields` with
`ClassLinker::LinkFieldsHelper::LinkFields` in ART's documentation.

Test: n/a (comment-only change)
Bug: 175869411
Change-Id: I9f74cad5d863d91dc0fc04e984350b1fc853d726
diff --git a/runtime/class_linker_test.cc b/runtime/class_linker_test.cc
index 6a6a388..0e7dee8 100644
--- a/runtime/class_linker_test.cc
+++ b/runtime/class_linker_test.cc
@@ -831,7 +831,7 @@
 
 // C++ fields must exactly match the fields in the Java classes. If this fails,
 // reorder the fields in the C++ class. Managed class fields are ordered by
-// ClassLinker::LinkFields.
+// ClassLinker::LinkFieldsHelper::LinkFields.
 TEST_F(ClassLinkerTest, ValidateFieldOrderOfJavaCppUnionClasses) {
   ScopedObjectAccess soa(Thread::Current());
   EXPECT_TRUE(ObjectOffsets().Check());