test_io.rb: fix typo

* test/ruby/test_io.rb (test_pid_after_close_read): fix typo.  pointed
  out by Peter Suschlik.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43133 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/ruby/test_io.rb

@@ -1409,7 +1409,7 @@ class TestIO < Test::Unit::TestCase
     assert_raise(IOError) { pipe.pid }
   end
 
-  def tesst_pid_after_close_read
+  def test_pid_after_close_read
     pid1 = pid2 = nil
     IO.popen(["echo", ""], "r+") do |io|
       pid1 = io.pid