Docs: fix typo (#171)

ICONS.md

@@ -8,7 +8,7 @@ Icons from the below listed sources can be used. When specifying an icon, use th
 
 * [Maki](https://labs.mapbox.com/maki-icons/) (`maki-`), map-specific icons from Mapbox
 * [Temaki](https://rapideditor.github.io/temaki/docs/) (`temaki-`), an expansion pack for Maki
-* [Röntgen](https://github.com/enzet/map-machine#r%C3%B6ntgen-icon-set) ([avaiable icons](https://github.com/openstreetmap/iD/tree/develop/svg/roentgen)) (`roentgen-`), part of the Map Machine project
+* [Röntgen](https://github.com/enzet/map-machine#r%C3%B6ntgen-icon-set) ([available icons](https://github.com/openstreetmap/iD/tree/develop/svg/roentgen)) (`roentgen-`), part of the Map Machine project
 * [Font Awesome](https://fontawesome.com/icons?d=gallery&m=free), thousands of general-purpose icons
     * There is a free and pro tier. You can use any icon from the free tier in the following styles:
         * [Solid](https://fontawesome.com/search?o=r&m=free&s=solid) (`fas-`)