fuck.bashrc: Added usage message to failure state

.bashrc.d/fuck.sh

@@ -3,6 +3,9 @@ function fuck {
   local nth_last=${1:-1}
   if [[ "$nth_last" =~ ^[0-9]+$ ]]; then
     sudo $(fc -ln -$nth_last -$nth_last)
+  else
+    echo "Usage: fuck [offset]"
+    return 1
   fi
 }