[contributing] Fix labels

Author: woojiahao

.github/ISSUE_TEMPLATE/exercise_discussion.md

@@ -2,7 +2,7 @@
 name: Exercise Discussions
 about: Want to create a new exercise for Git-Mastery? Discuss the details of the exercise with the Git-Mastery team first.
 title: "[Exercise Discussion]"
-labels: ["exercise-discussion"]
+labels: ["exercise discussion"]
 assignees: 
   - woojiahao
   - damithc

.github/ISSUE_TEMPLATE/request_exercise_repository.md

@@ -2,7 +2,7 @@
 name: Request for Exercise Repository
 about: Creating a new exercise for Git-Mastery and require a remote repository on the Git-Mastery organization? Request one here.
 title: "[Exercise Repository Request]"
-labels: exercise-repository-request
+labels: ["exercise repository request"]
 assignees: woojiahao
 ---