Fix plural typo in 'save' command help

The form "Save an images" is not correct.
Either "Save an image" or "Save images" work, but since
the save commands accepts multiple images, I chose the
latter.

Fixed in all places where I could grep "Save an image(s)".

Signed-off-by: Martin Mosegaard Amdisen <martin.amdisen@praqma.com>