Botox has created a dent in the middle of my head. What is the best way to rectify it? (Photo)

The depressed area that concerns you is high risk for fillers. You can leave it alone and it may improve over time or you can consider treatments such as SkinPen to help improve overall skin tone and texture. Typically 3-6 treatments are recommended for optimal results.

I have been injecting Botox for cosmetic reasons all over the face and neck since 1991 and have yet to encounter a single case of indentation in the forehead (or anywhere else, for that matter) since then.
On the other hand I have successfully treated steroid atrophy indentations of the forehead that appear quite similar to the indentation seen in the photo using an appropriate low viscosity, malleable, HA filler to immediately elevate and smooth it. You would be wise to make sure to consult a board certified cosmetic dermatologist filler expert and best of luck.

Whilst such treatments may 'unmask' underlying features of the forehead (e.g., muscle characteristics), the Botox shall 'not' have 'created' a dent in your forehead. I suspect that it is more likely that you had previously not noticed any contour abnormalities of your forehead (and are more critical after the Botox treatment). Take some photos and repeat the same in three months' time (in the same location with the same lighting), and then review at that point.