Bump kramdown from 2.4.0 to 2.5.1

Bumps [kramdown](https://github.com/gettalong/kramdown) from 2.4.0 to 2.5.1.
- [Release notes](https://github.com/gettalong/kramdown/releases)
- [Changelog](https://github.com/gettalong/kramdown/blob/master/doc/news.page)
- [Commits](https://github.com/gettalong/kramdown/commits)

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

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

Gemfile.lock

@@ -1,12 +1,12 @@
 GEM
   remote: https://rubygems.org/
   specs:
-    kramdown (2.4.0)
-      rexml
+    kramdown (2.5.1)
+      rexml (>= 3.3.9)
     power_assert (2.0.3)
     prettier_print (1.2.1)
     rake (13.2.1)
-    rexml (3.2.5)
+    rexml (3.3.9)
     rouge (4.5.1)
     syntax_tree (6.2.0)
       prettier_print (>= 1.2.0)