stan-reference: fix typo ('idexing' -> 'indexing')
stan-reference: fix typo ('idexing' -> 'indexing')

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