Fix state mv/rm -backup documentation

There is only one backup made, contrary to the help text. We may want to
implement that, but the documentation should match the current behavior.

command/state_mv.go

@@ -203,8 +203,7 @@ Options:
   -backup=PATH        Path where Terraform should write the backup for the original
                       state. This can't be disabled. If not set, Terraform
                       will write it to the same path as the statefile with
-                      a backup extension. This backup will be made in addition
-                      to the timestamped backup.
+                      a backup extension.
 
   -backup-out=PATH    Path where Terraform should write the backup for the destination
                       state. This can't be disabled. If not set, Terraform

command/state_rm.go

@@ -78,8 +78,7 @@ Options:
   -backup=PATH        Path where Terraform should write the backup
                       state. This can't be disabled. If not set, Terraform
                       will write it to the same path as the statefile with
-                      a backup extension. This backup will be made in addition
-                      to the timestamped backup.
+                      a backup extension.
 
   -state=statefile    Path to a Terraform state file to use to look
                       up Terraform-managed resources. By default it will