chore(deps): bump rack from 2.2.18 to 2.2.19 in /slate-docs (#549)

Author: dependabot[bot]

slate-docs/Gemfile.lock

@@ -93,7 +93,7 @@ GEM
     parslet (2.0.0)
     public_suffix (5.0.1)
     racc (1.8.1)
-    rack (2.2.18)
+    rack (2.2.19)
     rb-fsevent (0.11.2)
     rb-inotify (0.10.1)
       ffi (~> 1.0)