[SPARK-15225][SQL] Replace SQLContext with SparkSession in Encoder documentation

`Encoder`'s doc mentions `sqlContext.implicits._`. We should use `sparkSession.implicits._` instead now.

Only doc update.

Author: Liang-Chi Hsieh <simonh@tw.ibm.com>

Closes #13002 from viirya/encoder-doc.

sql/catalyst/src/main/scala/org/apache/spark/sql/Encoder.scala

@@ -29,13 +29,13 @@ import org.apache.spark.sql.types._
  * Used to convert a JVM object of type `T` to and from the internal Spark SQL representation.
  *
  * == Scala ==
- * Encoders are generally created automatically through implicits from a `SQLContext`, or can be
+ * Encoders are generally created automatically through implicits from a `SparkSession`, or can be
  * explicitly created by calling static methods on [[Encoders]].
  *
  * {{{
- *   import sqlContext.implicits._
+ *   import spark.implicits._
  *
- *   val ds = Seq(1, 2, 3).toDS() // implicitly provided (sqlContext.implicits.newIntEncoder)
+ *   val ds = Seq(1, 2, 3).toDS() // implicitly provided (spark.implicits.newIntEncoder)
  * }}}
  *
  * == Java ==
@@ -69,7 +69,7 @@ import org.apache.spark.sql.types._
 @Experimental
 @implicitNotFound("Unable to find encoder for type stored in a Dataset.  Primitive types " +
   "(Int, String, etc) and Product types (case classes) are supported by importing " +
-  "sqlContext.implicits._  Support for serializing other types will be added in future " +
+  "spark.implicits._  Support for serializing other types will be added in future " +
   "releases.")
 trait Encoder[T] extends Serializable {