add clarifying note about GELU

ch04/01_main-chapter-code/ch04.ipynb

@@ -667,7 +667,7 @@
    "metadata": {},
    "source": [
     "- As we can see, ReLU is a piecewise linear function that outputs the input directly if it is positive; otherwise, it outputs zero\n",
-    "- GELU is a smooth, non-linear function that approximates ReLU but with a non-zero gradient for negative values\n",
+    "- GELU is a smooth, non-linear function that approximates ReLU but with a non-zero gradient for negative values (except at approximately -0.75)\n",
     "\n",
     "- Next, let's implement the small neural network module, `FeedForward`, that we will be using in the LLM's transformer block later:"
    ]