Update "Migration Guides" redirect to point to 0.19. (#7837)

Update "Migration Guides" redirect to point to 0.19, so visitors see the
0.19 guide first instead of the 0.18 guide.

Author: kpreid

docs/content/reference/migration.md

@@ -1,5 +1,5 @@
 ---
 title: Migration Guides
 order: 1000
-redirect: reference/migration/migration-0-18
+redirect: reference/migration/migration-0-19
 ---