Correct constant names referenced in readme to reflect values used in code

ecac6b63 changed the constant names so they could be set independently of the Redis Object Cache plugin's options, but the readme wasn't updated accordingly.

readme.md

@@ -18,4 +18,4 @@ Requires WordPress 4.0, which should be released soon but isn’t currently suit
 
 By default, the plugin uses `127.0.0.1` and `6379` as the default host and port when creating a new client instance; the default database of `0` is also used. Three constants are provided to override these default values.
 
-Specify `WP_REDIS_BACKEND_HOST`, `WP_REDIS_BACKEND_PORT`, and `WP_REDIS_BACKEND_DB` to set the necessary, non-default connection values for your Redis instance.
+Specify `WP_REDIS_USER_SESSION_HOST`, `WP_REDIS_USER_SESSION_PORT`, and `WP_REDIS_USER_SESSION_DB` to set the necessary, non-default connection values for your Redis instance.