Update Templates.md to fix incorrect path structure

[topherfangio]

Description

Fix incorrect path structure in "Overriding Templates by Registering a Template Path" section.

Because of the YARD::Templates::Engine.register_template_path '/path/to/mytemplates' code above, we need a default subdirectory to contain the class/html/customsection.erb and class/setup.rb files.

The paragraph immediately after this even mentions the default/class structure, but I missed it since it wasn't in the tree structure.

Tested with yard 0.9.36 to verify.

Completed Tasks

• [x] I have read the Contributing Guide.
• [x] The pull request is complete (implemented / written).
• [x] Git commits have been cleaned up (squash WIP / revert commits).
• [x] I wrote tests and ran bundle exec rake locally (if code is attached to PR).