Update defaultpresets.xml to reflect new sac_scale value

[felagund]

An approved proposal added strolling to sac_scale: https://wiki.openstreetmap.org/wiki/Proposal:Add_strolling_to_sac_scale_and_some_further_refinements

Over time, the definition of the scale evolved, which was also codifiedby the approval process, so updating the wording to the current state of affairs.