test_fstring.rb: fix exception
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@56754 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
test_fstring.rb: fix exception
expected exception class. [ruby-dev:49867] [Bug #12923]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@56754 b2dd03c8-39d4-4d8f-98ff-823fe69b080e