Clarify reconnecting delay

Without clarification `delay` can mean two things: time to the next attempt (wrong) or time passed from the previous attempt (correct). The wrong interpretation is tempting because it makes easy to print "Trying to reconnect in {delay} ms."

Author: axelpale

README.md

@@ -180,7 +180,7 @@ emitted.
 
 `client` will emit `reconnecting` when trying to reconnect to the Redis server
 after losing the connection. Listeners are passed an object containing `delay`
-(in ms) and `attempt` (the attempt #) attributes.
+(in ms from the previous try) and `attempt` (the attempt #) attributes.
 
 ### "error"