doc: Use "connections" instead of "slots" to avoid confusion

The text was written before replication slots existed, but now "slot" is
best not used for anything else in the space of replication.

doc/src/sgml/ref/pg_basebackup.sgml

@@ -313,7 +313,7 @@ PostgreSQL documentation
             Stream the transaction log while the backup is created. This will
             open a second connection to the server and start streaming the
             transaction log in parallel while running the backup. Therefore,
-            it will use up two slots configured by the
+            it will use up two connections configured by the
             <xref linkend="guc-max-wal-senders"> parameter. As long as the
              client can keep up with transaction log received, using this mode
              requires no extra transaction logs to be saved on the master.