Changed api:versions config for v4 to point to v4 API link

refs a551970315
refs https://github.com/TryGhost/Team/issues/513

- This setting is used in url-utils (b17a965c88/core/shared/url-utils.js (L7)) which would create a `canary/admin` for `v4` with previous configuration. The links should link to one's explicit version if available, not to an alias like 'canary'

core/shared/config/overrides.json

@@ -67,8 +67,8 @@
                 "content": "canary/content"
             },
             "v4": {
-                "admin": "canary/admin",
-                "content": "canary/content"
+                "admin": "v4/admin",
+                "content": "v4/content"
             },
             "v3": {
                 "admin": "v3/admin",