fixes heading level for hypot

docs/ref/math.md

@@ -201,7 +201,7 @@ For derivatives, we therefore consider them both the same and calculate derivati
 
 `#!c++ T cbrt(T x)` computes the cubic root of `x`.
 
-### `hypot`
+#### `hypot`
 
 `#!c++ T hypot(T x, T y)` computes `sqrt(x*x + y*y)` without undue overflow or underflow at 
 intermediate stages of the computation.