Reduce marker spacing to 150mm

[WillB97]

Reducing the spacing allows each pair to be printed on a single landscape A4 page.

[WillB97]

 april_vision marker_generator TILE --num_columns 2 --num_rows 1 --marker_size 100 --page_size A4L --description_format 'Tin Can Rally - Marker {marker_id} ({marker_type})' --column_padding 25 --bottom_margin 20 --range 1,0,2,3,5,4,7,6,1,0,2,3,5,4,7,6

tin-can-rally-markers.pdf