Fixed custom issue template.

.github/ISSUE_TEMPLATE/custom.md

@@ -11,8 +11,9 @@ assignees: ''
 A clear and concise description of what you want to know.
 
 ### Which project is your bug related to?
-- [x] Client
-- [ ] ManagedClient
-- [ ] MQTTnet.Server standalone
-- [ ] Server
-- [ ] Generic
+<!-- Remove the items which don't apply from the following list -->
+- Client
+- ManagedClient
+- MQTTnet.Server standalone
+- Server
+- Generic