Replace the whole part on the inside. Lowe’s $10 10 minutes

This happens sometimes and is an easy fix. There should be a small diameter tube leading away from the valve assembly that goes into some type of standpipe (the tube is usually held in the standpipe by a wire clip, loop, etc.). The photo below shows such a tube going over to a black rectangular stand pipe. (This is a Kohler Cimmaron toilet, ~13 years old, but all toilet brands are similar.) The stand pipe can be any shape, round, rectangular, whatever, it doesn't matter, but what is important is that the stand pipe drains into the toilet bowl below that big flapper valve. While the valve assembly is refilling the main tank, the valve also pees a stream of water out the tube and into the stand pipe to refill the toilet bowl below. Your refill tube has probably come out of the standpipe holder and is peeing the water into the main tank instead of the standpipe. Simply place the tube back into whatever clip or retainer is there to hold it in the standpipe and you should be back in business.