Fix incorrect FrameAccess mode in TruffleBindingNodes

Author: eregon

src/main/java/org/truffleruby/core/binding/TruffleBindingNodes.java

@@ -42,7 +42,7 @@ public DynamicObject ofCaller() {
             final MaterializedFrame frame = Truffle.getRuntime().iterateFrames(frameInstance -> {
                 if (frameCount.get() == 2) {
                     sourceSection.set(frameInstance.getCallNode().getEncapsulatingSourceSection());
-                    return frameInstance.getFrame(FrameInstance.FrameAccess.READ_WRITE).materialize();
+                    return frameInstance.getFrame(FrameInstance.FrameAccess.MATERIALIZE).materialize();
                 } else {
                     frameCount.set(frameCount.get() + 1);
                     return null;