update saveImaged docstrings (#7076)

Fixes #6884 .

### Description

Documentation didn't indicate dcm files support. After checking dcm
seems to be the only extension whose doc was missing.

### 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`.
- [x] In-line docstrings updated.
- [x] Documentation updated, tested `make html` command in the `docs/`
folder.

Signed-off-by: vgrau98 <victor.grau93@gmail.com>

Author: vgrau98

monai/transforms/io/dictionary.py

@@ -198,7 +198,7 @@ class SaveImaged(MapTransform):
                     Handled by ``folder_layout`` instead, if ``folder_layout`` is not ``None``.
         output_postfix: a string appended to all output file names, default to ``trans``.
                         Handled by ``folder_layout`` instead, if ``folder_layout`` is not ``None``.
-        output_ext: output file extension name, available extensions: ``.nii.gz``, ``.nii``, ``.png``.
+        output_ext: output file extension name, available extensions: ``.nii.gz``, ``.nii``, ``.png``, ``.dcm``.
                     Handled by ``folder_layout`` instead, if ``folder_layout`` not ``None``.
         resample: whether to resample image (if needed) before saving the data array,
             based on the ``spatial_shape`` (and ``original_affine``) from metadata.