Clarify what is written and read in IO#popen spec

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@60568 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

spec/ruby/core/io/popen_spec.rb

@@ -22,7 +22,7 @@ describe "IO.popen" do
 
   it "raises IOError when writing a read-only pipe" do
     @io = IO.popen(ruby_cmd('puts "foo"'), "r")
-    lambda { @io.write('foo') }.should raise_error(IOError)
+    lambda { @io.write('bar') }.should raise_error(IOError)
     @io.read.should == "foo\n"
   end
 end