fix toc landing pages (#5581)

Author: jeff-allen-mongo

snooty.toml

@@ -181,6 +181,7 @@ toc_landing_pages = [
    "/reference/security",
    "/reference/sharding",
    "/reference/stable-api",
+   "/reference/write-concern",
    "/release-notes/2.4",
    "/release-notes/2.6",
    "/release-notes/3.0",