fixed doxygen equation tags for stan/math/constants.hpp
fixed doxygen equation tags for stan/math/constants.hpp

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