docs: add missing cache_sleep to constructor options list

Signed-off-by: disqualifier <dev@disqualifier.me>

README.md

@@ -150,6 +150,7 @@ through.
 - `ephemeral` — send/edit ephemerally
 - `page_text` — format string for the jump button label
 - `emojis` — override navigation emojis
+- `cache_sleep` — seconds to wait after cache-priming before refreshing (default 1.0)
 
 ## Subclassing