Fixed #4029: test the network every 500 iterations, not 1000 iterations

examples/mnist/readme.md

@@ -248,7 +248,7 @@ These messages tell you the details about each layer, its connections and its ou
     I1203 solver.cpp:36] Solver scaffolding done.
     I1203 solver.cpp:44] Solving LeNet
 
-Based on the solver setting, we will print the training loss function every 100 iterations, and test the network every 1000 iterations. You will see messages like this:
+Based on the solver setting, we will print the training loss function every 100 iterations, and test the network every 500 iterations. You will see messages like this:
 
     I1203 solver.cpp:204] Iteration 100, lr = 0.00992565
     I1203 solver.cpp:66] Iteration 100, loss = 0.26044