Fix link to Structure in docs (#247)

Author: mcmire

docs/contributors/architecture/introduction.md

@@ -9,4 +9,4 @@ these guides heavily skew toward that topic,
 and you can start with a [guide to how RSpec works if you like](./how-rspec-works.md).
 But you can also find [details around the diff engine](./how-super-diff-works.md).
 Finally, if you're curious about the files in this project and how they're organized,
-feel free to consult the [structure](./structure) document.
+feel free to consult the [structure](./structure.md) document.