Vulnerability issue in kramdown dependency (#8131)

Summary:
GitHub has detected that a package defined in the
docs/Gemfile.lock file of the facebook/rocksdb repository contains a
security vulnerability.
This patch fixes it by upgrading the version of kramdown to 2.3.1

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8131

Reviewed By: jay-zhuang

Differential Revision: D27418776

Pulled By: akankshamahajan15

fbshipit-source-id: 0a4b0b85922b9958afcbc44560584701b1c6c82d

docs/Gemfile.lock

@@ -70,7 +70,7 @@ GEM
       jekyll-theme-time-machine (= 0.1.1)
       jekyll-titles-from-headings (= 0.5.3)
       jemoji (= 0.12.0)
-      kramdown (= 2.3.0)
+      kramdown (= 2.3.1)
       kramdown-parser-gfm (= 1.1.0)
       liquid (= 4.0.3)
       mercenary (~> 0.3)
@@ -196,7 +196,7 @@ GEM
       gemoji (~> 3.0)
       html-pipeline (~> 2.2)
       jekyll (>= 3.0, < 5.0)
-    kramdown (2.3.0)
+    kramdown (2.3.1)
       rexml
     kramdown-parser-gfm (1.1.0)
       kramdown (~> 2.0)