[SQL] fix function description mistake

Sample code in the description of SchemaRDD.where is not correct

Author: Jacky Li <jacky.likun@gmail.com>

Closes apache#3344 from jackylk/patch-6 and squashes the following commits:

62cd126 [Jacky Li] [SQL] fix function description mistake

sql/core/src/main/scala/org/apache/spark/sql/SchemaRDD.scala

@@ -307,7 +307,7 @@ class SchemaRDD(
    * Filters tuples using a function over the value of the specified column.
    *
    * {{{
-   *   schemaRDD.sfilter('a)((a: Int) => ...)
+   *   schemaRDD.where('a)((a: Int) => ...)
    * }}}
    *
    * @group Query