merge: Fix check on number of arguments
merge: Fix check on number of arguments

I changed my mind mi-way and the old check was preventing from having 3
or more input files.

Signed-off-by: Damien Lespiau <64bd3cb94f359c1a3ce68dae5e26b40578526277@intel.com>

dlespiau/covertool,dlespiau/covertool