Fix typo in SSIMMetric (#7612)

Fixes #7610


### Types of changes
<!--- Put an `x` in all the boxes that apply, and remove the not
applicable items -->
- [x] Non-breaking change (fix or new feature that would not break
existing functionality).
- [ ] Breaking change (fix or new feature that would cause existing
functionality to change).
- [ ] New tests added to cover the changes.
- [ ] Integration tests passed locally by running `./runtests.sh -f -u
--net --coverage`.
- [ ] Quick tests passed locally by running `./runtests.sh --quick
--unittests --disttests`.
- [ ] In-line docstrings updated.
- [ ] Documentation updated, tested `make html` command in the `docs/`
folder.

Signed-off-by: YunLiu <55491388+KumoLiu@users.noreply.github.com>

Author: KumoLiu

monai/metrics/regression.py

@@ -303,7 +303,7 @@ def _compute_metric(self, y_pred: torch.Tensor, y: torch.Tensor) -> torch.Tensor
 
         if self.spatial_dims == 3 and dims != 5:
             raise ValueError(
-                f"y_pred should have 4 dimensions (batch, channel, height, width, depth) when using {self.spatial_dims}"
+                f"y_pred should have 5 dimensions (batch, channel, height, width, depth) when using {self.spatial_dims}"
                 f" spatial dimensions, got {dims}."
             )