Update cv command

[vpchung]

Proposed Changes

• adding the required -o <output image> to the -cv command example on the docs
• only print the success message if an updated image is successful and an output image filename is provided

Additional Context

Currently, when I use the -cv command without specifying an output image name, I get a ImageFileWriter error as well as a misleading success message, e.g.

$ captk -i Downloads/BraTS2021_00001.nii.gz -cv 4,3
Error occurred while trying to write the image '': /usr/local/vsts/_work/1/s/bin/ITK-source/Modules/IO/ImageBase/include/itkImageFileWriter.hxx:123:
itk::ERROR: ImageFileWriter(0x7fb2c5104d50): No filename was specified
Create Mask completed.

[sarthakpati]

LGTM