doc: fix example in repl documentation

Define the context property r.context.m, not r.m.

PR-URL: https://github.com/nodejs/node/pull/8469


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>