Fix typo in source docs

[timofurrer]

This change fixes a typo in the docs for the source

[nedbat]

Thanks! You set the record for highest commits-to-characters-changed ratio!

[timofurrer]

@nedbat omg, I know. codespaces git-tooling is nothing like the command line git workflow I'm used to 😝 Luckily, there is squashing 💃

Thanks for the awesome work Ned!