[MINOR] a typo: coalesce

Author: Daoyuan Wang <daoyuan.wang@intel.com>

Closes #5482 from adrian-wang/typo and squashes the following commits:

e65ef6f [Daoyuan Wang] typo

sql/core/src/main/scala/org/apache/spark/sql/execution/GeneratedAggregate.scala

@@ -97,7 +97,7 @@ case class GeneratedAggregate(
         val currentSum = AttributeReference("currentSum", calcType, nullable = true)()
         val initialValue = Literal.create(null, calcType)
 
-        // Coalasce avoids double calculation...
+        // Coalesce avoids double calculation...
         // but really, common sub expression elimination would be better....
         val zero = Cast(Literal(0), calcType)
         val updateFunction = Coalesce(