Fix typo in StringIO#initialize's specs

Author: andrykonchin

spec/ruby/library/stringio/initialize_spec.rb

@@ -192,7 +192,7 @@
 # NOTE: Synchronise with core/io/new_spec.rb (core/io/shared/new.rb)
 describe "StringIO#initialize when passed keyword arguments" do
   it "sets the mode based on the passed :mode option" do
-    io = StringIO.new("example", "r")
+    io = StringIO.new("example", mode: "r")
     io.closed_read?.should be_false
     io.closed_write?.should be_true
   end