Add real Google Analytics ID

Summary:
This adds the actual RocksDB Google Analytics ID that will be used when
we switch over from WP to GH-pages

Test Plan: visual

Reviewers: IslamAbdelRahman, lgalanis, sdong

Reviewed By: sdong

Subscribers: sdong, andrewkr, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D63429

docs/_config.yml

@@ -5,7 +5,7 @@ tagline: A persistent key-value store for fast storage environments
 description: >
   RocksDB is an embeddable persistent key-value store for fast storage.
 fbappid: "1615782811974223"
-gacode: "UA-12345-6"
+gacode: "UA-49459723-1"
 # baseurl determines the subpath of your site. For example if you're using an
 # organisation.github.io/reponame/ basic site URL, then baseurl would be set
 # as "/reponame/" but leave blank if you have a top-level domain URL