HADOOP-15181. Typo in SecureMode.md

Signed-off-by: Akira Ajisaka <aajisaka@apache.org>
(cherry picked from commit 6e27b20b4d695ed94bd44c60f191cfbb5cdb32e2)
(cherry picked from commit ab482ebe73fbe24bf5242e705793e224f0f73500)

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

@@ -575,7 +575,7 @@ or implicitly set to "simple":
 
 Needless to say, an application so configured cannot talk to a secure Hadoop cluster.
 
-#### `--verifyshortname <principal>`: validate the short name of a principal
+#### `--verifyshortname <principal>`: validate the short name of a principal
 
 This verifies that the short name of a principal contains neither the `"@"`
 nor `"/"` characters.