Invalid link parsing - broken link if ends in '.md'

[EugZol]

ruby 3.0.2p107 (2021-07-07 revision 0db68f0233) [x86_64-linux]
rdoc 6.3.1

Create the following ruby file in some folder:

# https://github.com/ruby/ruby/blob/master/README.md
class Klass
end

Run rdoc in that folder.

Run grep ruby/ruby doc/Klass.html.

Expected: link href is https://github.com/ruby/ruby/blob/master/README.md

Actual: link href is https://github_com/ruby/ruby/blob/master/README_md.html