Add missing option doc for 'packer fmt' (#12944)

website/content/docs/commands/fmt.mdx

@@ -52,4 +52,7 @@ $ cat my-template.pkr.hcl | packer fmt -
 - `-write=false` - Don't write formatting changes to source files
   (always disabled if using -check)
 
-- `-` - read formatting changes from stdin and write them to stdout.
+- `-` - read formatting changes from stdin and write them to stdout.
+
+- `recursive` Also process files in subdirectories. By default, only the
+  given directory (or current directory) is processed.