Grouting a very very small area
The best solution is to get more of the same material and install it just like you did for the rest of the grout. This will keep you happy in the following years as you know you did the right thing.

Now it may be possible to patch this spot up with some sink sealer (bathroom tub / sink sealer such as DAP brand) but I can guess that it will look like a hack and in future you will always look at it and know you could have done better.

The other thing to consider is that a small batch of the grout material is not likely going to cost a whole lot more than having to purchase some type of sealer material. It may even be possible that you can even find a grout repair kit that has the material in a small bag instead of a whole wheel barrow load type of bag.