stan-reference: fix typo in MCMC Sampling (54.2)
stan-reference: fix typo in MCMC Sampling (54.2)

"there is" was duplicated in the same sentence

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