[SPARK-7811] Fix typo on slf4j configuration on metrics.properties.tem…

Fix minor typo on metrics.properties.template where slf4j is incorrectly spelled as sl4j.

Author: Judy Nash <judynash@microsoft.com>

Closes #6362 from judynash/master and squashes the following commits:

c644875 [Judy Nash] SPARK-7811: Fix typo on slf4j configuration on metrics.properties.template

conf/metrics.properties.template

@@ -126,9 +126,9 @@
 #*.sink.slf4j.class=org.apache.spark.metrics.sink.Slf4jSink
 
 # Polling period for Slf4JSink
-#*.sink.sl4j.period=1
+#*.sink.slf4j.period=1
 
-#*.sink.sl4j.unit=minutes
+#*.sink.slf4j.unit=minutes
 
 
 # Enable jvm source for instance master, worker, driver and executor