There's a mistake in the Using the url_key method section

[rokf]

This part has a mistake.

- The value of any parameter in params is a string then it is inserted into the generated path as is.
+ If the value of any parameter in params is a string, then it is inserted into the generated path as is.

Source: https://leafo.net/lapis/reference/actions.html#request-object-methods/request:url_for/using-the-url-key-method

[rokf]

I've included the fix in leafo/lapis#769