lib/rdoc/parser/ruby.rb: Avoid .chars.to_a.last

The code creates a lot of useless objects.
Instead, using a regexp is shorter and faster.

lib/rdoc/parser/ruby.rb

@@ -1779,9 +1779,9 @@ class RDoc::Parser::Ruby < RDoc::Parser
           while tk and (:on_comment == tk[:kind] or :on_embdoc == tk[:kind]) do
             comment_body = retrieve_comment_body(tk)
             comment += comment_body
-            comment += "\n" unless "\n" == comment_body.chars.to_a.last
+            comment << "\n" unless comment_body =~ /\n\z/
 
-            if comment_body.size > 1 && "\n" == comment_body.chars.to_a.last then
+            if comment_body.size > 1 && comment_body =~ /\n\z/ then
               skip_tkspace_without_nl # leading spaces
             end
             tk = get_tk