[SQL] Fix typo in DataframeWriter doc

## What changes were proposed in this pull request?

The format of none should be consistent with other compression codec(\`snappy\`, \`lz4\`) as \`none\`.

## How was this patch tested?

This is a typo.

Author: GuoChenzhao <chenzhao.guo@intel.com>

Closes #18758 from gczsjdy/typo.

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

@@ -499,7 +499,7 @@ final class DataFrameWriter[T] private[sql](ds: Dataset[T]) {
    * <ul>
    * <li>`compression` (default is the value specified in `spark.sql.parquet.compression.codec`):
    * compression codec to use when saving to file. This can be one of the known case-insensitive
-   * shorten names(none, `snappy`, `gzip`, and `lzo`). This will override
+   * shorten names(`none`, `snappy`, `gzip`, and `lzo`). This will override
    * `spark.sql.parquet.compression.codec`.</li>
    * </ul>
    *