Update README.md

Author: thompson-tomo

README.md

@@ -150,7 +150,7 @@ This is useful CI environments where you want to check if your docs are up to da
 
 You can print to stdout by using the `-s` or `--stdout` option.
 
-This option is only applicable when specifying the filename doctoc is to run on, if specifying a folder the dry run option should be used.
+This option is only applicable when specifying a single filename which doctoc is to run on, if specifying a folder or multiple files the dry run option should be used.
 
 ### Only update existing ToC