this is a manifest.f4m link and i have only found a .m3u8 url
rtmpdump is not possible with that
but you can download this video with ffmpeg
Code:
ffmpeg -i "http://medicitvod-vh.akamaihd.net/i/abr/2973/2973_full_en_OSM_Hamelin_def_hd2_,360,510,720,1030,1450,2110,3010,.mp4.csmil/index_6_av.m3u8?null=" -c copy output.ts