[doc] Fix value of eps in MNIST property.

doc/mnist.rst

@@ -141,7 +141,7 @@ samples, we can define a goal and check the property
   goal robustness:
     let nn = read_neural_network "nets/MNIST_256_2.onnx" ONNX in
     let dataset = read_dataset "csv/mnist_test.csv" CSV in
-    let eps = (0.375:t) in
+    let eps = (0.0100000000000000002081668171172168513294309377670288085937500000:t) in
     robust nn dataset eps