docs: replace dummy with placeholder in example comments (#7064)

* docs: replace 'dummy' with 'placeholder' in example comments

Use more inclusive language in code comments across example files.
The term 'dummy' can be replaced with the clearer, more neutral
term 'placeholder' without any change in meaning.

Files changed:
- examples/ejs/index.js
- examples/route-middleware/index.js
- examples/auth/index.js

* ci: re-trigger coverage after transient coveralls.io error

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Co-authored-by: stuckvgn <[email protected]>