fixed minor typo in appendix for Phi_approx
fixed minor typo in appendix for Phi_approx

stan-dev/stan,stan-dev/stan,stan-dev/stan,stan-dev/stan,stan-dev/math,stan-dev/math,stan-dev/stan,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math