fix typos. add a reference to deliverable and stretch for design artifact

CONTRIBUTING.md

@@ -168,7 +168,7 @@ hits. They are not always necessary, but very convenient.
 
 If you are an expert in a particular area, it makes it easier to find issues to
 work on. You can also subscribe to those labels to receive an email each time an
-issue is labelled with a subject label corresponding to your expertise.
+issue is labeled with a subject label corresponding to your expertise.
 
 Examples of subject labels are ~wiki, ~"container registry", ~ldap, ~api,
 ~issues, ~"merge requests", ~labels, and ~"container registry".
@@ -315,10 +315,10 @@ Once an issue has been worked on and is ready for development, a UXer applies th
 
 The UX team has a special type label called ~"design artifact". This label indicates that the final output 
 for an issue is a UX solution/design. The solution will be developed by frontend and/or backend in a subsequent milestone. 
-Any issue labelled ~"design artifact" should not also be labeled ~"frontend" or ~"backend" since no development is 
+Any issue labeled ~"design artifact" should not also be labeled ~"frontend" or ~"backend" since no development is 
 needed until the solution has been decided.
 
-~"design artifact" issues are like any other issue and should contain a milestone label when scheduled in the current milestone.
+~"design artifact" issues are like any other issue and should contain a milestone label, ~"Deliverable" or ~"Stretch", when scheduled in the current milestone.
 
 Once the ~"design artifact" issue has been completed, the UXer removes the ~"design artifact" label and applies the ~"UX ready" label. The Product Manager can use the 
 existing issue or decide to create a whole new issue for the purpose of development.