Update bootstrap.rst

fix a copy/paste typo

Author: dellamowica

testing/bootstrap.rst

@@ -29,7 +29,7 @@ You can modify this file to add custom logic:
     +     // executes the "php bin/console cache:clear" command
     +     passthru(sprintf(
     +         'APP_ENV=%s php "%s/../bin/console" cache:clear --no-warmup',
-    +         $_ENV['BOOTSTRAP_CLEAR_CACHE_ENV'],
+    +         $_ENV['APP_ENV'],
     +         __DIR__
     +     ));
     + }