src: fix typo rval to value

rval never existed, it was added as that in 077f9d7293

Fixes: https://github.com/nodejs/node/issues/9001
PR-URL: https://github.com/nodejs/node/pull/9023
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>

src/node.cc

@@ -2544,7 +2544,7 @@ static void EnvSetter(Local<String> property,
     SetEnvironmentVariableW(key_ptr, reinterpret_cast<WCHAR*>(*val));
   }
 #endif
-  // Whether it worked or not, always return rval.
+  // Whether it worked or not, always return value.
   info.GetReturnValue().Set(value);
 }