instructors.md: Fix typo "used" -> "uses"

instructors.md

@@ -7,7 +7,7 @@ subtitle: Instructor's Guide
 
 We follow two scientists, Dracula and Wolfman, collaborating in a project. In
 the first part of the lesson, Dracula is working by himself in his laptop and
-used Git locally. In the second part of the lesson, he uploads his local repo
+uses Git locally. In the second part of the lesson, he uploads his local repo
 to GitHub ands add Wolfman as a collaborator. With both working in the same
 project, conflicts arise and we see how to handle them with Git. The lesson
 closes with a part on Open Science that is somewhat detached from the story.