ffmpeg add-on: fix typo.

src/add-ons/media/plugins/ffmpeg/gfx_util.cpp

@@ -152,7 +152,7 @@ pixfmt_to_string(int pixFormat)
 {
 	const char* name = av_get_pix_fmt_name((enum PixelFormat)pixFormat);
 	if (name == NULL)
-		return "(unknownn)";
+		return "(unknown)";
 	return name;
 }