Clarify exception message

src/Composer/Config.php

@@ -180,7 +180,7 @@ class Config
                 if ($env = getenv('COMPOSER_DISCARD_CHANGES')) {
                     if (!in_array($env, array('stash', 'true', 'false', '1', '0'), true)) {
                         throw new \RuntimeException(
-                            "Invalid value for 'discard-changes': {$this->config[$key]}, expected 1, 0, true, false or stash"
+                            "Invalid value for COMPOSER_DISCARD_CHANGES: {$this->config[$key]}, expected 1, 0, true, false or stash"
                         );
                     }
                     if ('stash' === $env) {