Move i3-snapshot to screenshot

i3-snapshot → screenshot

@@ -26,7 +26,7 @@ do
             FILENAME="$OPTARG"
             ;;
         [?])
-            echo "i3-snapshot [ -? ] [ -r | -w ] [ -f filename.png ]" >&2
+            echo "screenshot [ -? ] [ -r | -w ] [ -f filename.png ]" >&2
             exit 1
             ;;
     esac