Try to fix broken gitbook reference docs

Author: iv-p

docs/reference.md

@@ -41,7 +41,7 @@ variables:
 
 Commands are a familiar way to manipulate variables. They allow you to run shell commands and use their output. There are practically no limitations as to what you can do.
 
-The syntax for commands is very similar to the syntax of variables, but instead of using `{{` and `}}` as as delimiters, it uses `{%` and `%}`. For example `{$ echo $ENV_VARIABLE $}`
+The syntax for commands is very similar to the syntax of variables, but instead of using `{{` and `}}` as as delimiters, it uses `{% shell cmd %}`. For example `{$ echo $ENV_VARIABLE $}`
 
 ### Executables