note that SPLASH_SLOT_POLICY value isn't a string

put a little note about what value should `SPLASH_SLOT_POLICY` setting have

Author: iAnanich

README.rst

@@ -107,7 +107,7 @@ Put them into your ``settings.py`` if you want to change the defaults:
 * ``SPLASH_LOG_400`` is ``True`` by default - it instructs to log all 400 errors
   from Splash. They are important because they show errors occurred
   when executing the Splash script. Set it to ``False`` to disable this logging.
-* ``SPLASH_SLOT_POLICY`` is ``scrapy_splash.SlotPolicy.PER_DOMAIN`` by default.
+* ``SPLASH_SLOT_POLICY`` is ``scrapy_splash.SlotPolicy.PER_DOMAIN`` (as object, not just a string) by default.
   It specifies how concurrency & politeness are maintained for Splash requests,
   and specify the default value for ``slot_policy`` argument for
   ``SplashRequest``, which is described below.