chore(deps): bump nkf from 0.2.0 to 0.3.0 in /example

Bumps [nkf](https://github.com/ruby/nkf) from 0.2.0 to 0.3.0.
- [Release notes](https://github.com/ruby/nkf/releases)
- [Commits](ruby/nkf@v0.2.0...v0.3.0)

---
updated-dependencies:
- dependency-name: nkf
  dependency-version: 0.3.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

example/Gemfile.lock

@@ -89,7 +89,7 @@ GEM
     nanaimo (0.3.0)
     nap (1.1.0)
     netrc (0.11.0)
-    nkf (0.2.0)
+    nkf (0.3.0)
     prism (1.9.0)
     public_suffix (4.0.7)
     rexml (3.4.4)