update cat_chns arg's docstring (#5051)

Signed-off-by: KumoLiu <yunl@nvidia.com>

Fixes #5046 .

### Description
update `cat_chns` arg's docstring in basic_unet.

### Status
**Ready**

### 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: KumoLiu <yunl@nvidia.com>

Author: KumoLiu

monai/networks/nets/basic_unet.py

@@ -124,7 +124,7 @@ def __init__(
         Args:
             spatial_dims: number of spatial dimensions.
             in_chns: number of input channels to be upsampled.
-            cat_chns: number of channels from the decoder.
+            cat_chns: number of channels from the encoder.
             out_chns: number of output channels.
             act: activation type and arguments.
             norm: feature normalization type and arguments.