HDFS-11478. Update EC commands in HDFSCommands.md. Contributed by Yiqun Lin.

hadoop-hdfs-project/hadoop-hdfs/src/site/markdown/HDFSCommands.md

@@ -436,8 +436,9 @@ Runs the diskbalancer CLI. See [HDFS Diskbalancer](./HDFSDiskbalancer.html) for
 Usage:
 
        hdfs ec [generic options]
-         [-setPolicy [-p <policyName>] <path>]
-         [-getPolicy <path>]
+         [-setPolicy -policy <policyName> -path <path>]
+         [-getPolicy -path <path>]
+         [-unsetPolicy -path <path>]
          [-listPolicies]
          [-usage [cmd ...]]
          [-help [cmd ...]]
@@ -446,6 +447,7 @@ Usage:
 |:---- |:---- |
 |-setPolicy| Set a specified ErasureCoding policy to a directory|
 |-getPolicy| Get ErasureCoding policy information about a specified path|
+|-unsetPolicy| Unset an ErasureCoding policy set by a previous call to "setPolicy" on a directory |
 |-listPolicies| Lists all supported ErasureCoding policies|
 
 Runs the ErasureCoding CLI. See [HDFS ErasureCoding](./HDFSErasureCoding.html#Administrative_commands) for more information on this command.