Update Gemfile.lock

Author: lauren-mills

docs/Gemfile.lock

@@ -69,7 +69,7 @@ GEM
  jekyll-theme-time-machine (= 0.1.1)
  jekyll-titles-from-headings (= 0.5.1)
  jemoji (= 0.10.2)
- kramdown (= 1.17.0)
+ kramdown (= 2.3.1)
  liquid (= 4.0.0)
  listen (= 3.1.5)
  mercenary (~> 0.3)