This is a normal situation, nothing to worry about it. It happens when child put her head in a certain position for longer duration.
Next Steps
Try to change her head position regularly. Also use baby head pillow, they have soft cushion in centre which help to keep head round.
Health Tips
Also, during bathing of child with warm water, try to massage gently in soft circular motion to make it round for 2-3 minutes. Keep one thing in mind it's not a one day process, it will take time maximum one month. But yes, it's reversible.