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

---------

Co-authored-by: stuckvgn <stuckvgn@users.noreply.github.com>

Author: stuckvgn

examples/auth/index.js

@@ -38,7 +38,7 @@ app.use(function(req, res, next){
   next();
 });
 
-// dummy database
+// placeholder database
 
 var users = {
   tj: { name: 'tj' }

examples/ejs/index.js

@@ -35,7 +35,7 @@ app.use(express.static(path.join(__dirname, 'public')));
 // ex: res.render('users.html').
 app.set('view engine', 'html');
 
-// Dummy users
+// Placeholder users
 var users = [
   { name: 'tobi', email: 'tobi@learnboost.com' },
   { name: 'loki', email: 'loki@learnboost.com' },

examples/route-middleware/index.js

@@ -15,7 +15,7 @@ var app = express();
 //     curl http://localhost:3000/user/1/edit (unauthorized since this is not you)
 //     curl -X DELETE http://localhost:3000/user/0 (unauthorized since you are not an admin)
 
-// Dummy users
+// Placeholder users
 var users = [
   { id: 0, name: 'tj', email: 'tj@vision-media.ca', role: 'member' }
   , { id: 1, name: 'ciaran', email: 'ciaranj@gmail.com', role: 'member' }