Style layer four headlines properly

[ISibboI]

I use four layers of headlines in the README.md of my crate.
But the fourth layer is just shown as simple text without any styling. On github it looks as it should, not that big but visible as a headline by being bold.

The headlines in question are The Aggregation Operator, The Assignment Operator and The Expression Chaining Operator on the crate's page. Compare with the same version on github.