GP Covariance Functions: minor tweak

[joshuaalley]

Summary:

This is really small, but I think that the documentation for Gaussian Processes uses the old cov_exp_quad, instead of gp_exp_quad_cov and the like.

Description:

For instance, the kernel is written as: matrix[N, N] K = cov_exp_quad(x, alpha, rho);

In the STAN Matrix Operations Docs, the kernels are gp_exp_quad_cov.

Additional Information:

Using cov_exp_quad now throws compilation errors on cmdstan, so people who follow the user guide on may have some problems.

Current Version:

v2.18.0

[bob-carpenter]

Thanks for submitting---we will fix.