Update .github/ISSUE_TEMPLATE/lesson_contribution.md

fix small typo

Author: JaroCamphuijsen

.github/ISSUE_TEMPLATE/lesson_contribution.md

@@ -8,7 +8,7 @@ assignees: ''
 ---
 
 **Short description of the diagnostic**
-Add a short description of the mterial that you would like to add.
+Add a short description of the material that you would like to add.
 
 **Branch and pull request**
 Once you've started working, add the branch (and pull request) link.