[Minor] Fix doc typo for describing primitiveTerm effectiveness condition

It should be `true` instead of `false`?

Author: Liang-Chi Hsieh <viirya@gmail.com>

Closes #4762 from viirya/doc_fix and squashes the following commits:

2e37482 [Liang-Chi Hsieh] Fix doc.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/codegen/CodeGenerator.scala

@@ -121,7 +121,7 @@ abstract class CodeGenerator[InType <: AnyRef, OutType <: AnyRef] extends Loggin
    * @param nullTerm A term that holds a boolean value representing whether the expression evaluated
    *                 to null.
    * @param primitiveTerm A term for a possible primitive value of the result of the evaluation. Not
-   *                      valid if `nullTerm` is set to `false`.
+   *                      valid if `nullTerm` is set to `true`.
    * @param objectTerm A possibly boxed version of the result of evaluating this expression.
    */
   protected case class EvaluatedExpression(