Contract references in instructions for setting up Emacs for michelson are obsolete

This page: http://tezos.gitlab.io/user/various.html#environment-for-writing-michelson-contracts

refers to ./src/bin_client/test/contracts/attic/id.tz (twice) which no longer exists.

this contract now lives in ./tests_python/contracts_alpha/attic/id.tz.

in addition, we can use the :src: role (see docs/README.rst) to raise a warning should this new location also become obsolete in the future. To do that we would have to work a link to the contract into the text, but should be feasible.