[SPARK-1826] fix the head notation of package object dsl

Author: wangfei <scnbwf@yeah.net>

Closes #765 from scwf/dslfix and squashes the following commits:

d2d1a9d [wangfei] Update package.scala
66ff53b [wangfei] fix the head notation of package object dsl

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/dsl/package.scala

@@ -31,26 +31,29 @@ import org.apache.spark.sql.catalyst.types._
  * A collection of implicit conversions that create a DSL for constructing catalyst data structures.
  *
  * {{{
- *  scala> import catalyst.dsl._
+ *  scala> import org.apache.spark.sql.catalyst.dsl.expressions._
  *
  *  // Standard operators are added to expressions.
+ *  scala> import org.apache.spark.sql.catalyst.expressions.Literal
  *  scala> Literal(1) + Literal(1)
- *  res1: catalyst.expressions.Add = (1 + 1)
+ *  res0: org.apache.spark.sql.catalyst.expressions.Add = (1 + 1)
  *
  *  // There is a conversion from 'symbols to unresolved attributes.
  *  scala> 'a.attr
- *  res2: catalyst.analysis.UnresolvedAttribute = 'a
+ *  res1: org.apache.spark.sql.catalyst.analysis.UnresolvedAttribute = 'a
  *
  *  // These unresolved attributes can be used to create more complicated expressions.
  *  scala> 'a === 'b
- *  res3: catalyst.expressions.Equals = ('a = 'b)
+ *  res2: org.apache.spark.sql.catalyst.expressions.Equals = ('a = 'b)
  *
  *  // SQL verbs can be used to construct logical query plans.
- *  scala> TestRelation('key.int, 'value.string).where('key === 1).select('value).analyze
- *  res4: catalyst.plans.logical.LogicalPlan =
- *  Project {value#1}
- *   Filter (key#0 = 1)
- *    TestRelation {key#0,value#1}
+ *  scala> import org.apache.spark.sql.catalyst.plans.logical._
+ *  scala> import org.apache.spark.sql.catalyst.dsl.plans._
+ *  scala> LocalRelation('key.int, 'value.string).where('key === 1).select('value).analyze
+ *  res3: org.apache.spark.sql.catalyst.plans.logical.LogicalPlan =
+ *  Project [value#3]
+ *   Filter (key#2 = 1)
+ *    LocalRelation [key#2,value#3], []
  * }}}
  */
 package object dsl {