Fix oob index in sparse example

A sparse matrix with shape [4, 10] is being assigned an element at location [7, 0]. This changes that to the valid [3, 0].

site/en/guide/sparse_tensor.ipynb

@@ -256,7 +256,7 @@
         "                       values=[31, 2], \n",
         "                       dense_shape=[4, 10])\n",
         "\n",
-        "st_b = tf.sparse.SparseTensor(indices=[[0, 2], [7, 0]],\n",
+        "st_b = tf.sparse.SparseTensor(indices=[[0, 2], [3, 0]],\n",
         "                       values=[56, 38],\n",
         "                       dense_shape=[4, 10])\n",
         "\n",