Add missing -c argument to watcher documentation

source/blog/2023-01-06-opal-1-7-ruby-3-2.html.md

@@ -17,7 +17,7 @@ Opal v1.7 introduces the `--watch` flag, which allows you to automatically recom
 To use the `--watch` flag, simply pass it as an argument when running the `opal` command. For example:
 
 ```shell
-opal --output=main.js --watch main.rb
+opal --output=main.js --watch -c main.rb
 ```
 
 This will start the Opal compiler and watch for any changes to the `main.rb` file. When a change is detected, the compiler will automatically recompile the code.