[MINOR] Avoid the 'latest' link that might vary per release in functions.scala's comment

## What changes were proposed in this pull request?

This PR propose to address apache#21318 (comment) comment.

This is rather a nit but looks we better avoid to update the link for each release since it always points the latest (it doesn't look like worth enough updating release guide on the other hand as well).

## How was this patch tested?

N/A

Author: hyukjinkwon <[email protected]>

Closes apache#21907 from HyukjinKwon/minor-fix.

Author: HyukjinKwon

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

@@ -44,8 +44,8 @@ import org.apache.spark.util.Utils
  *
  * Spark also includes more built-in functions that are less common and are not defined here.
  * You can still access them (and all the functions defined here) using the `functions.expr()` API
- * and calling them through a SQL expression string. You can find the entire list of functions for
- * the latest version of Spark at https://spark.apache.org/docs/latest/api/sql/index.html.
+ * and calling them through a SQL expression string. You can find the entire list of functions
+ * at SQL API documentation.
  *
  * As an example, `isnan` is a function that is defined here. You can use `isnan(col("myCol"))`
  * to invoke the `isnan` function. This way the programming language's compiler ensures `isnan`