Fixed typo in sql.functions

## What changes were proposed in this pull request?

I fixed a typo in the Scaladoc for the method `def struct(cols: Column*): Column`. 'retained' was misspelt as 'remained'.

## How was this patch tested?
Before:

Creates a new struct column.
   If the input column is a column in a `DataFrame`, or a derived column expression
   that is named (i.e. aliased), its name would be **remained** as the StructField's name,
   otherwise, the newly generated StructField's name would be auto generated as
   `col` with a suffix `index + 1`, i.e. col1, col2, col3, ...

After:

   Creates a new struct column.
   If the input column is a column in a `DataFrame`, or a derived column expression
   that is named (i.e. aliased), its name would be **retained** as the StructField's name,
   otherwise, the newly generated StructField's name would be auto generated as
   `col` with a suffix `index + 1`, i.e. col1, col2, col3, ...

Author: sujithjay <sujith@logistimo.com>

Closes #18254 from sujithjay/fix-typo.

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

@@ -1210,7 +1210,7 @@ object functions {
   /**
    * Creates a new struct column.
    * If the input column is a column in a `DataFrame`, or a derived column expression
-   * that is named (i.e. aliased), its name would be remained as the StructField's name,
+   * that is named (i.e. aliased), its name would be retained as the StructField's name,
    * otherwise, the newly generated StructField's name would be auto generated as
    * `col` with a suffix `index + 1`, i.e. col1, col2, col3, ...
    *