Change doc to reflect opset export version

lib/nir/node.mli

@@ -29,7 +29,7 @@
 
     [descr] describes several operations. When an operation shares the same name
     as an ONNX operation, it follows the standard defined in the ONNX IR v8 and
-    ONNX Opset v22 standards, described here:
+    ONNX Opset v13 standards, described here:
     https://onnx.ai/onnx/operators/index.html.
 
     Nodes only require their inputs: it is assumed that a node only returns one