CLI.hs: fix double spaces in option help strings

Some help strings with continuation backslashes ('\') were providing a
space both before and after the backslash, resulting in double spaces in
help output. Provide it only after the backslash, which fixes the issue and
is consistent with the rest of the file.

Signed-off-by: Dato Simó <dato@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>