Repairing ceramic tile
One method, if you have spare tiles left over from the original installation, is to remove the damaged tile and the grout around the tile. Removal of the damaged tile needs to be done carefully so as to not damage adjacent tile. One recommendation is to drill holes in the tile and then break it up in smallish pieces with a chisel between the holes. Then after cleaning out the whole area under the tile a spare one can then be re-set in place and re-grouted. 

A patch may be possible by filling the divot with a white epoxy. Doing so on a vertical surface will require some finnesse. Fixing by this method will require the hole to be clean, very dry and all loose chips and crumbs to be removed. (From looking at your picture it appears that there is an additional small part of the glazed surface that would have to be removed). 

Note that you can also purchase ceramic tile chip and crack repair kits.