Revert "docs: update docs to use HttpClientModule instead of HttpModule (#22727)"

This reverts commit b5533e0ee5.

aio/content/examples/ngmodules/src/app/app.module.ts

@@ -1,7 +1,7 @@
 import { BrowserModule } from '@angular/platform-browser';
 import { NgModule } from '@angular/core';
 import { FormsModule } from '@angular/forms';
-import { HttpClientModule } from '@angular/common/http';
+import { HttpModule } from '@angular/http';
 
 /* App Root */
 import { AppComponent } from './app.component';