fix incorrect sample config (`1` instead of `true`)

reported in #721

Change-Id: I0e569ec7404604a28c00aabd6d154b9bfa7200ff

etc/rtpengine-recording.sample.conf

@@ -30,10 +30,10 @@ table = 0
 # mp3_bitrate = 24000
 
 ### mix participating sources into a single output
-# output-mixed = 1
+# output-mixed = true
 
 ### create one output file for each source
-# output-single = 1
+# output-single = true
 
 ### mysql configuration for db storage
 # mysql-host = localhost