Bump rake from 12.1.0 to 12.3.3 in /example_app

Bumps [rake](https://github.com/ruby/rake) from 12.1.0 to 12.3.3.
- [Release notes](https://github.com/ruby/rake/releases)
- [Changelog](https://github.com/ruby/rake/blob/master/History.rdoc)
- [Commits](ruby/rake@v12.1.0...v12.3.3)

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

example_app/Gemfile.lock

@@ -46,7 +46,7 @@ GEM
       rack
     rack-test (0.7.0)
       rack (>= 1.0, < 3)
-    rake (12.1.0)
+    rake (12.3.3)
     rb-fsevent (0.10.2)
     rb-inotify (0.9.10)
       ffi (>= 0.5.0, < 2)