contexts.md: Update migration example output (#3919)
contexts.md: Update migration example output (#3919)

To go with the change in https://github.com/elixir-ecto/ecto_sql/pull/240.
nshafer/phoenix,nshafer/phoenix,phoenixframework/phoenix,eksperimental/phoenix,eksperimental/phoenix,michalmuskala/phoenix,michalmuskala/phoenix,optikfluffel/phoenix,rafbgarcia/phoenix,optikfluffel/phoenix,0x00evil/phoenix,0x00evil/phoenix,optikfluffel/phoenix,phoenixframework/phoenix,rafbgarcia/phoenix,phoenixframework/phoenix,0x00evil/phoenix,nshafer/phoenix,rafbgarcia/phoenix