scanf.rb: fix rescue modifier

* lib/scanf.rb (IO#scanf): fix mistaken use of rescue modifier.
  a patch by Mon_Ouie at [ruby-core:52813].  [Bug #7940]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44196 b2dd03c8-39d4-4d8f-98ff-823fe69b080e