Added az 15 migration guide to index page (#3758)

Author: mikefrobbins

docs-conceptual/azps-15.0.0/index.yml

@@ -52,8 +52,8 @@ landingContent:
             url: whats-new-overview.md
           - text: Release notes
             url: release-notes-azureps.md
-          - text: Az 14.0.0 migration guide
-            url: migrate-az-14.0.0.md
+          - text: Az 15.0.0 migration guide
+            url: migrate-az-15.0.0.md
           - text: Upcoming breaking changes
             url: upcoming-breaking-changes.md
           - text: Impact of MFA in automation