use less load-balancy words for round robin transport

Author: pableu

mailer.rst

@@ -231,8 +231,8 @@ The round-robin transport starts with a *randomly* selected transport and
 then switches to the next available transport for each subsequent email.
 
 As with the failover transport, round-robin retries deliveries until
-a transport succeeds (or all failed). In contrast to the failover transport,
-it  *balances* the load between all its transports.
+a transport succeeds (or all fail). In contrast to the failover transport,
+it *spreads* the load a all its transports.
 
 .. versionadded:: 5.1