lib/rdoc/markup/: Remove wrong call to `=~` against Array

`@res` is an Array, so `@res =~ /\n\z/` calls `Object#=~` which always
returns nil.
I guess it should be `@res.last =~ /\n\z/`, but the change causes test
failures.

This bug was found during work for removal of `Object#=~`.
[Feature #15231]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65985 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/rdoc/markup/to_markdown.rb

@@ -131,7 +131,7 @@ class RDoc::Markup::ToMarkdown < RDoc::Markup::ToRdoc
       @res << part
     end
 
-    @res << "\n" unless @res =~ /\n\z/
+    @res << "\n"
   end
 
   ##

lib/rdoc/markup/to_rdoc.rb

@@ -234,7 +234,7 @@ class RDoc::Markup::ToRdoc < RDoc::Markup::Formatter
       @res << part
     end
 
-    @res << "\n" unless @res =~ /\n\z/
+    @res << "\n"
   end
 
   ##