Fix typo in documentation comment

[ulrikstrid]

Without calling the s function you will get a type error as you won't have a Parser.

The newline at the end was added by github.

[process-bot]

Thanks for the pull request! Make sure it satisfies this checklist. My human colleagues will appreciate it!

Here is what to expect next, and if anyone wants to comment, keep these things in mind.

[evancz]

Thanks!