fixed L_cov_exp_quad_ARD function signature, fixes #675

Author: avehtari

src/stan-users-guide/gaussian-processes.qmd

@@ -555,7 +555,7 @@ covariance matrix called `L_cov_exp_quad_ARD`.
 
 ```stan
 functions {
-  matrix L_cov_exp_quad_ARD(vector[] x,
+  matrix L_cov_exp_quad_ARD(array[] vector x,
                             real alpha,
                             vector rho,
                             real delta) {