misc/opts.c: change usage string for vector arguments
misc/opts.c: change usage string for vector arguments

We now show the specific way we want to specifiy the arguments in vec2
and vec3.

sidward/bart,sidward/bart,sidward/bart,mrirecon/bart,mrirecon/bart,mrirecon/bart,sidward/bart,mrirecon/bart,sidward/bart,mrirecon/bart