[SPARK-6767][SQL] Fixed Query DSL error in spark sql Readme

Fixed the  following error
query.where('key > 30).select(avg('key)).collect()
<console>:43: error: value > is not a member of Symbol
              query.where('key > 30).select(avg('key)).collect()

Author: Tijo Thomas <tijoparacka@gmail.com>

Closes #5415 from tijoparacka/ERROR_SQL_DATAFRAME_EXAMPLE and squashes the following commits:

234751e [Tijo Thomas] Fixed Query DSL error in spark sql Readme

sql/README.md

@@ -56,6 +56,6 @@ res2: Array[org.apache.spark.sql.Row] = Array([238,val_238], [86,val_86], [311,v
 
 You can also build further queries on top of these `DataFrames` using the query DSL.
 ```
-scala> query.where('key > 30).select(avg('key)).collect()
+scala> query.where(query("key") > 30).select(avg(query("key"))).collect()
 res3: Array[org.apache.spark.sql.Row] = Array([274.79025423728814])
 ```