Correct build for ruby 2.7.
Ruby 2.6 has following files available:
1 lrwxr-xr-x 1 root wheel 6 Oct 13 10:45 /usr/local/bin/rdoc -> rdoc26
5 -rwxr-xr-x 1 root wheel 518 Oct 13 10:44 /usr/local/bin/rdoc26

Ruby 2.7 has:
6 -rwxr-xr-x 1 root wheel 518 Nov 23 07:28 /usr/local/bin/rdoc

So there is no need to have a version included for the rdoc command.