executables named ⌜nmm2x⌝ should show correct usage info

[anderslundstedt]

We currently have the following:

$ nmm2txt
usage: nmm nmm2txt [-h] path
nmm nmm2txt: error: the following arguments are required: path
$ nmm2xml
usage: nmm nmm2xml [-h] path
nmm nmm2xml: error: the following arguments are required: path

We should have ⌜nmm2x⌝ instead of ⌜nmm nmm2x⌝:

$ nmm2txt
usage: nmm2txt [-h] path
nmm nmm2txt: error: the following arguments are required: path
$ nmm2xml
usage: nmm2xml [-h] path
nmm nmm2xml: error: the following arguments are required: path