fix Dir.mktmpdir doc.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@13086 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/tmpdir.rb

@@ -54,7 +54,7 @@ class Dir
   # the prefix "d" is used.
   #
   # The directory is created under Dir.tmpdir or
-  # the optional second argument <i>tmpdir</i> if given.
+  # the optional second argument <i>tmpdir</i> if non-nil value is given.
   #
   # If a block is given,
   # it is yielded with the path of the directory.