Correct ch.4 MNIST: set threshold to 0.0 when not using sigmoid (#461)

* Correct ch.4 MNIST: set threshold to 0.0 when not using sigmoid

* Also change 0.5 to 0.0 in the text

Co-authored-by: Ben van der Burgh <ben@metabolic.nl>

04_mnist_basics.ipynb

@@ -3833,7 +3833,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "Let's check our accuracy. To decide if an output represents a 3 or a 7, we can just check whether it's greater than 0.5, so our accuracy for each item can be calculated (using broadcasting, so no loops!) with:"
+    "Let's check our accuracy. To decide if an output represents a 3 or a 7, we can just check whether it's greater than 0.0, so our accuracy for each item can be calculated (using broadcasting, so no loops!) with:"
    ]
   },
   {
@@ -3859,7 +3859,7 @@
     }
    ],
    "source": [
-    "corrects = (preds>0.5).float() == train_y\n",
+    "corrects = (preds>0.0).float() == train_y\n",
     "corrects"
    ]
   },