fix: docs/Gemfile.lock to reduce vulnerabilities (#224)

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-RUBY-ADDRESSABLE-1316242

Author: snyk-bot

docs/Gemfile.lock

@@ -1,20 +1,20 @@
 GEM
   remote: https://rubygems.org/
   specs:
-    addressable (2.7.0)
+    addressable (2.8.0)
       public_suffix (>= 2.0.2, < 5.0)
     colorator (1.1.0)
-    concurrent-ruby (1.1.8)
+    concurrent-ruby (1.1.9)
     em-websocket (0.5.2)
       eventmachine (>= 0.12.9)
       http_parser.rb (~> 0.6.0)
     eventmachine (1.2.7)
     eventmachine (1.2.7-x64-mingw32)
-    ffi (1.14.2)
-    ffi (1.14.2-x64-mingw32)
+    ffi (1.15.3)
+    ffi (1.15.3-x64-mingw32)
     forwardable-extended (2.6.0)
     http_parser.rb (0.6.0)
-    i18n (1.8.7)
+    i18n (1.8.10)
       concurrent-ruby (~> 1.0)
     jekyll (4.2.0)
       addressable (~> 2.4)
@@ -48,7 +48,7 @@ GEM
     kramdown-parser-gfm (1.1.0)
       kramdown (~> 2.0)
     liquid (4.0.3)
-    listen (3.4.1)
+    listen (3.5.1)
       rb-fsevent (~> 0.10, >= 0.10.3)
       rb-inotify (~> 0.9, >= 0.9.10)
     mercenary (0.4.0)
@@ -59,8 +59,8 @@ GEM
     pathutil (0.16.2)
       forwardable-extended (~> 2.6)
     public_suffix (4.0.6)
-    rake (13.0.3)
-    rb-fsevent (0.10.4)
+    rake (13.0.4)
+    rb-fsevent (0.11.0)
     rb-inotify (0.10.1)
       ffi (~> 1.0)
     rexml (3.2.5)
@@ -93,4 +93,4 @@ DEPENDENCIES
   wdm (~> 0.1.1)
 
 BUNDLED WITH
-   2.2.9
+   2.1.4