fix double t_srs argument

On line 179 of the Makefile there were two -t_srs arguments. That looks wrong compared to the other changes made in https://github.com/interactivethings/swiss-maps/commit/38a7a87d4bc2e6c94167430833822a8205e6ab3f .

I have not tested this. This is just a change to make it look similar to the other entries.