Tweak test method and variable name

Co-authored-by: Sutou Kouhei <[email protected]>

Author: tompng

test/stringio/test_stringio.rb

@@ -922,9 +922,9 @@ def test_encoding_write
     assert_equal("abc".encode("utf-32be"), s.string)
   end
 
-  def test_write_str_modify
-    seek_pos = [0, 2, 4, 6]
-    seek_pos.each do |pos|
+  def test_write_code_range
+    seek_poses = [0, 2, 4, 6]
+    seek_poses.each do |pos|
       s = StringIO.new('abcd')
       s.seek(pos)
       s.write("\xff\xff\xff")