[SPARK-6511] [docs] Fix example command in hadoop-provided docs.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes apache#6766 from vanzin/SPARK-6511 and squashes the following commits:

49f0f67 [Marcelo Vanzin] [SPARK-6511] [docs] Fix example command in hadoop-provided docs.

docs/hadoop-provided.md

@@ -21,6 +21,6 @@ export SPARK_DIST_CLASSPATH=$(hadoop classpath)
 export SPARK_DIST_CLASSPATH=$(/path/to/hadoop/bin/hadoop classpath)
 
 # Passing a Hadoop configuration directory
-export SPARK_DIST_CLASSPATH=$(hadoop classpath --config /path/to/configs)
+export SPARK_DIST_CLASSPATH=$(hadoop --config /path/to/configs classpath)
 
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