Fix typo of file path [ci skip]

Patch by: yuuji.yaginuma <yuuji.yaginuma@gmail.com>

https://github.com/ruby/ruby/pull/1900
[Fix GH-1900]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63775 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/drb/drb.rb

@@ -360,7 +360,7 @@ module DRb
   # drb remains valid only while that object instance remains alive
   # within the server runtime.
   #
-  # For alternative mechanisms, see DRb::TimerIdConv in rdb/timeridconv.rb
+  # For alternative mechanisms, see DRb::TimerIdConv in drb/timeridconv.rb
   # and DRbNameIdConv in sample/name.rb in the full drb distribution.
   class DRbIdConv