fix: fix the README's grep command

Author: ROMemories

README.md

@@ -19,7 +19,7 @@ You also have to activate the preprocessor, put this in your `book.toml` file:
 > :warning: This preprocessor presents a security risk, as arbitrary commands can be run. Be careful with the commands you run.
 > To list all the commands that will be run within an mdbook, you can run the following command:
 > ```sh
-> grep -r '<!-- cmdrun' . | sed 's/.*<!-- cmdrun \(.*\) -->.*/\1/'
+> grep -r '<!--\s*cmdrun' . | sed 's/.*<!--\s*cmdrun \(.*\) -->.*/\1/'
 > ``````