rubyfmt: Use language-specific heredoc delimiters

Author: issyl0

Formula/r/rubyfmt.rb

@@ -46,14 +46,14 @@ def install
   end
 
   test do
-    (testpath/"test.rb").write <<~EOS
+    (testpath/"test.rb").write <<~RUBY
       def foo; 42; end
-    EOS
-    expected = <<~EOS
+    RUBY
+    expected = <<~RUBY
       def foo
         42
       end
-    EOS
+    RUBY
     assert_equal expected, shell_output("#{bin}/rubyfmt -- #{testpath}/test.rb")
   end
 end