docs: fix typo in injected variable name (angular#23315)

The service injected is `ValueService`, however the name of the variable
does not reflect that. It's actually confusing since it's the name of
the `class` being created.

PR Close angular#23315

aio/content/examples/testing/src/app/demo/demo.ts

@@ -37,8 +37,8 @@ export class ValueService {
 // #docregion MasterService
 @Injectable()
 export class MasterService {
-  constructor(private masterService: ValueService) { }
-  getValue() { return this.masterService.getValue(); }
+  constructor(private valueService: ValueService) { }
+  getValue() { return this.valueService.getValue(); }
 }
 // #enddocregion MasterService