[ruby/psych] Update to squiggly heredocs in the file

https://github.com/ruby/psych/commit/42b43de997

test/psych/test_hash.rb

@@ -102,13 +102,13 @@ module Psych
     end
 
     def test_ref_append
-      hash = Psych.unsafe_load(<<-eoyml)
----
-foo: &foo
-  hello: world
-bar:
-  <<: *foo
-eoyml
+      hash = Psych.unsafe_load(<<~eoyml)
+        ---
+        foo: &foo
+          hello: world
+        bar:
+          <<: *foo
+      eoyml
       assert_equal({"foo"=>{"hello"=>"world"}, "bar"=>{"hello"=>"world"}}, hash)
     end
 
@@ -157,11 +157,11 @@ eoyml
         pend "This Ruby implementation doesn't support string deduplication"
       end
 
-      hashes = Psych.load(<<-eoyml)
----
-- unique_identifier: 1
-- unique_identifier: 2
-eoyml
+      hashes = Psych.load(<<~eoyml)
+        ---
+        - unique_identifier: 1
+        - unique_identifier: 2
+      eoyml
 
       assert_same hashes[0].keys.first, hashes[1].keys.first
     end