Bump rdoc from 6.11.0 to 6.13.1

Bumps [rdoc](https://github.com/ruby/rdoc) from 6.11.0 to 6.13.1.
- [Release notes](https://github.com/ruby/rdoc/releases)
- [Changelog](https://github.com/ruby/rdoc/blob/master/History.rdoc)
- [Commits](ruby/rdoc@v6.11.0...v6.13.1)

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

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

Author: dependabot[bot]

Gemfile.lock

@@ -94,7 +94,7 @@ GEM
     rb-fsevent (0.11.2)
     rb-inotify (0.11.1)
       ffi (~> 1.0)
-    rdoc (6.11.0)
+    rdoc (6.13.1)
       psych (>= 4.0.0)
     regexp_parser (2.10.0)
     rspec (3.13.0)
@@ -154,7 +154,7 @@ GEM
       strscan (>= 1.0.0)
       terminal-table (>= 2, < 5)
       uri (>= 0.12.0)
-    stringio (3.1.5)
+    stringio (3.1.6)
     strong_json (2.1.2)
     strscan (3.1.2)
     tempfile (0.3.1)