Update RFC-00xx-special-functions.md

Author: 0x00b1

RFC-00xx-special-functions.md

@@ -1941,9 +1941,11 @@ spherical_hankel_h_2(
 ) -> Tensor
 ```
 
-Hankel function of the second kind:
+Spherical Hankel function of the second kind:
+
+$$h_{n}^{2}(z)=\frac{\sqrt{\frac{\pi}{2}}H_{n+\frac{1}{2}}^{2}(z)}{\sqrt{z}},$$
 
-$$h_{n}^{2}(z) = j_{n}(z) - iy_{n}(z).$$
+where $H_{n}^{2}$ is the Hankel function of the second kind.
 
 ##### Parameters