Spec's scatter example has incorrect dimensions (#2876)

In scatter's example, the dimensions for `%update` don't match between
the commented input and the MLIR.


https://github.com/openxla/stablehlo/blob/96acdcb7724f4a9eec6d2e5af2597b0750c13948/docs/spec.md?plain=1#L5739-L5749


https://github.com/openxla/stablehlo/blob/96acdcb7724f4a9eec6d2e5af2597b0750c13948/docs/spec.md?plain=1#L5763

```python
>>> jnp.array([
...            [
...             [[1, 1], [1, 1], [1, 1]],
...             [[1, 1], [1, 1], [1, 1]]
...            ],
...            [
...             [[1, 1], [1, 1], [1, 1]],
...             [[1, 1], [1, 1], [1, 1]]
...            ]
...           ]).shape
(2, 2, 3, 2)
```

Author: kentslaney

docs/spec.md

@@ -5738,12 +5738,12 @@ undefined.
 //                   ]
 // %update: [
 //           [
-//            [[1, 1], [1, 1], [1, 1]],
-//            [[1, 1], [1, 1], [1, 1]]
+//            [[[1, 1], [1, 1]], [[1, 1], [1, 1]], [[1, 1], [1, 1]]],
+//            [[[1, 1], [1, 1]], [[1, 1], [1, 1]], [[1, 1], [1, 1]]]
 //           ],
 //           [
-//            [[1, 1], [1, 1], [1, 1]],
-//            [[1, 1], [1, 1], [1, 1]]
+//            [[[1, 1], [1, 1]], [[1, 1], [1, 1]], [[1, 1], [1, 1]]],
+//            [[[1, 1], [1, 1]], [[1, 1], [1, 1]], [[1, 1], [1, 1]]]
 //           ]
 //          ]
 %result = "stablehlo.scatter"(%input, %scatter_indices, %update) ({