Fix d_out code comment in bonus materials (#715)

ch03/02_bonus_efficient-multihead-attention/mha-implementations.ipynb

@@ -689,7 +689,7 @@
     "    def __init__(self, d_in, d_out, num_heads, context_length, dropout=0.0, qkv_bias=False):\n",
     "        super().__init__()\n",
     "\n",
-    "        assert d_out % num_heads == 0, \"embed_dim is indivisible by num_heads\"\n",
+    "        assert d_out % num_heads == 0, \"d_out is indivisible by num_heads\"\n",
     "\n",
     "        self.num_heads = num_heads\n",
     "        self.context_length = context_length\n",
@@ -1002,7 +1002,7 @@
     "    def __init__(self, d_in, d_out, num_heads, context_length, dropout=0.0, qkv_bias=False):\n",
     "        super().__init__()\n",
     "\n",
-    "        assert d_out % num_heads == 0, \"embed_dim is indivisible by num_heads\"\n",
+    "        assert d_out % num_heads == 0, \"d_out is indivisible by num_heads\"\n",
     "\n",
     "        self.num_heads = num_heads\n",
     "        self.context_length = context_length\n",