Have Facebook link point to RocksDB on FB

Summary: ...not just the main FB landing page

Test Plan: visual

Reviewers: lgalanis, sdong

Reviewed By: sdong

Subscribers: andrewkr, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D63969

docs/_data/nav.yml

@@ -19,5 +19,5 @@
   category: blog
 
 - title: Facebook
-  href: http://www.facebook.com/
+  href: https://www.facebook.com/groups/rocksdb.dev/
   category: facebook