6849122: G1: Typo introduced during implementation of the parallel refinement

Summary: Typo fix
Reviewed-by: jcoomes

src/share/vm/gc_implementation/g1/concurrentG1Refine.cpp

@@ -111,7 +111,7 @@ ConcurrentG1Refine::~ConcurrentG1Refine() {
     for (int i = 0; i < _n_threads; i++) {
       delete _threads[i];
     }
-    FREE_C_HEAP_ARRAY(ConcurrentG1RefineThread*, _n_threads);
+    FREE_C_HEAP_ARRAY(ConcurrentG1RefineThread*, _threads);
   }
 }