Allow logger to do lazy evaluation of format strings

Change-Id: I39d26cdd5b85a61a06fd8c7a9d0a046e398819bd
diff --git a/pySim-trace.py b/pySim-trace.py
index eb29ed1..91f50a3 100755
--- a/pySim-trace.py
+++ b/pySim-trace.py
@@ -182,7 +182,7 @@
 
     opts = option_parser.parse_args()
 
-    logger.info('Opening source %s...' % opts.source)
+    logger.info('Opening source %s...', opts.source)
     if opts.source == 'gsmtap-udp':
         s = GsmtapApduSource(opts.bind_ip, opts.bind_port)
     elif opts.source == 'rspro-pyshark-pcap':