MARR documentation fix

[Uthar]

Hello, thanks for making this library, it's very useful in game development.

I found an error in the documentation, which should say SIMPLE-ARRAY (because SIMPLE-VECTOR always has element type T):

https://github.com/Shinmera/3d-matrices/blob/b4f5a58299f8437fad2d77f0879c374e3430f4b5/documentation.lisp#L551

Best regards, Kasper