refactor(docs-infra): fix docs examples for tslint rules related to underscores in variable names (#38143)

This commit updates the docs examples to be compatible with the
`variable-name` tslint rule without requiring the
`allow-leading-underscore` and `allow-trailing-underscore` options.

This is in preparation of updating the docs examples `tslint.json` to
match the one generated for new Angular CLI apps in a future commit.

PR Close #38143

aio/content/examples/ngmodules/src/app/greeting/user.service.ts

@@ -12,7 +12,6 @@ export class UserServiceConfig {
 })
 export class UserService {
   id = nextId++;
-  private _userName = 'Sherlock Holmes';
 
   // #docregion ctor
   constructor(@Optional() config?: UserServiceConfig) {
@@ -25,4 +24,5 @@ export class UserService {
     const suffix = this.id > 1 ? ` times ${this.id}` : '';
     return this._userName + suffix;
   }
+  private _userName = 'Sherlock Holmes';  // tslint:disable-line: variable-name
 }