Fix typo in "Structuring Tests" page

changing `theen` to `then`

PR-URL: #725
Credit: @yurm04
Close: #725
Reviewed-by: @isaacs

docs/src/content/docs/structure/index.md

@@ -145,7 +145,7 @@ and tests the basic API.  When the first bug is found, you can add a
 failing test at `test/bug-description.js`, and then update the code to make
 the test pass.  When features are added, you can add example code at
 `test/feature-description.js` that demonstrates using the feature, and
-theen update the code to make it pass by implementing the feature.
+then update the code to make it pass by implementing the feature.
 
 Over time, you might end up with something like this: