Fix wording in error log message, to be consistent with other messages ("IO...

Fix wording in error log message, to be consistent with other messages ("IO thread" -> "I/O thread").

sql/slave.cc

@@ -5565,7 +5565,7 @@ static int queue_event(Master_info* mi,const char* buf, ulong event_len)
       char str_buf[128];
       String str(str_buf, sizeof(str_buf), system_charset_info);
       mi->rli.until_gtid_pos.to_string(&str);
-      sql_print_information("Slave IO thread stops because it reached its"
+      sql_print_information("Slave I/O thread stops because it reached its"
                             " UNTIL master_gtid_pos %s", str.c_ptr_safe());
       mi->abort_slave= true;
     }