fix a typo [ci skip]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@50175 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

tool/downloader.rb

@@ -91,7 +91,7 @@ class Downloader
         options['If-Modified-Since'] = since
       end
     end
-    options['Accept-Encoding'] = '*' # to deisable Net::HTTP::GenericRequest#decode_content
+    options['Accept-Encoding'] = '*' # to disable Net::HTTP::GenericRequest#decode_content
     options
   end