stan-reference: fix typo in HMC (56.1)
stan-reference: fix typo in HMC (56.1)

"desnity" should be "density"

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