doc: fix example in repl documentation

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

PR-URL: #8469
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/repl.md

@@ -118,7 +118,7 @@ const repl = require('repl');
 var msg = 'message';
 
 const r = repl.start('> ');
-Object.defineProperty(r, 'm', {
+Object.defineProperty(r.context, 'm', {
   configurable: false,
   enumerable: true,
   value: msg