at approx scale for WMTS

Related to issue [#1629] (https://github.com/geoadmin/mf-geoadmin3/issues/1629). 

In API doc : Simply replace string "Scale at 96 dpi" by "Approx. scale at 96 dpi per zoom level" in the table if I understood well.

[Test link] (http://mf-chsdi3.dev.bgdi.ch/kan_api_doc_approx_scale/services/sdiservices.html#wmts)