Update docs/Gemfile.lock to fix github warning (#4480)

Summary:
Fix security warning from github: https://nvd.nist.gov/vuln/detail/CVE-2018-17567
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4480

Reviewed By: gfosco

Differential Revision: D10316443

Pulled By: yiwu-arbug

fbshipit-source-id: 34555c6f5774d779734b664b9baa70bd4209175f

docs/Gemfile.lock

@@ -25,7 +25,7 @@ GEM
     github-pages (104)
       activesupport (= 4.2.7)
       github-pages-health-check (= 1.2.0)
-      jekyll (= 3.3.0)
+      jekyll (>= 3.8.4)
       jekyll-avatar (= 0.4.2)
       jekyll-coffeescript (= 1.0.1)
       jekyll-feed (= 0.8.0)
@@ -56,7 +56,7 @@ GEM
       activesupport (>= 2)
       nokogiri (~> 1.8.2)
     i18n (0.7.0)
-    jekyll (3.3.0)
+    jekyll (3.8.4)
       addressable (~> 2.4)
       colorator (~> 1.0)
       jekyll-sass-converter (~> 1.0)