HDFS-17325. Fix the documentation of fs expunge command in FileSystemShell.md. (#6413) Contributed by liuguanghua.

Reviewed-by: Ayush Saxena <ayushsaxena@apache.org>
Signed-off-by: Shilun Fan <slfan1989@apache.org>

hadoop-common-project/hadoop-common/src/site/markdown/FileSystemShell.md

@@ -289,7 +289,7 @@ rather than the default filesystem and checkpoint is created.
 For example
 
 ```
-hadoop fs -expunge --immediate -fs s3a://landsat-pds/
+hadoop fs -expunge -immediate -fs s3a://landsat-pds/
 ```
 
 Refer to the