Update description about --backend-config for tfmigrate plan

command/plan.go

@@ -123,13 +123,8 @@ Arguments:
 
 Options:
   --config                 A path to tfmigrate config file
-  --backend-config=path    Configuration to be merged with what is in the
-                           configuration file's 'backend' block. This can be
-                           either a path to an HCL file with key/value
-                           assignments (same format as terraform.tfvars) or a
-                           'key=value' format, and can be specified multiple
-                           times. The backend type must be in the configuration
-                           itself.
+  --backend-config=path    A backend configuration, a path to backend configuration file or key=value format backend configuraion.
+                           This option is passed to terraform init when switching backend to remote.
 
   [Deprecated]
   --out=path