minor doc fix for Row.scala

## What changes were proposed in this pull request?

minor doc fix for "getAnyValAs" in class Row

## How was this patch tested?

None.

(If this patch involves UI changes, please attach a screenshot; otherwise, remove this)

Author: buzhihuojie <ren.weiluo@gmail.com>

Closes #15452 from david-weiluo-ren/minorDocFixForRow.

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

@@ -462,7 +462,7 @@ trait Row extends Serializable {
   def mkString(start: String, sep: String, end: String): String = toSeq.mkString(start, sep, end)
 
   /**
-   * Returns the value of a given fieldName.
+   * Returns the value at position i.
    *
    * @throws UnsupportedOperationException when schema is not defined.
    * @throws ClassCastException when data type does not match.