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

This commit updates the docs examples to be compatible with the
`no-shadowed-variable` and `variable-name` tslint rules.

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/dependency-injection/src/app/providers.component.ts

@@ -237,7 +237,7 @@ export class Provider9Component implements OnInit {
 import { Optional } from '@angular/core';
 // #enddocregion import-optional
 
-let some_message = 'Hello from the injected logger';
+let someMessage = 'Hello from the injected logger';
 
 @Component({
   selector: 'provider-10',
@@ -249,7 +249,7 @@ export class Provider10Component implements OnInit {
   // #docregion provider-10-ctor
   constructor(@Optional() private logger?: Logger) {
     if (this.logger) {
-      this.logger.log(some_message);
+      this.logger.log(someMessage);
     }
   }
   // #enddocregion provider-10-ctor