use rdoc compatible with system ruby

despite https://rubygems.org/gems/rdoc/versions/7.2.0 saying that ruby 2.6 is supported, in fact it is not:
- ruby/rdoc#1618
- ruby/rdoc#1615

Author: CamJN

Gemfile.lock

@@ -35,7 +35,7 @@ GEM
       logger
       prism (>= 1.6.0)
       tsort
-    rdoc (7.2.0)
+    rdoc (7.1.0)
       erb
       psych (>= 4.0.0)
       tsort