Add missing plugins for tracing the sample app (open-telemetry#1509)

When I followed this guide I got some errors after running 

```sh
$ npm install \
  @opentelemetry/core \
  @opentelemetry/node \
  @opentelemetry/plugin-http
```

The errors:
PluginLoader#load: could not load plugin @opentelemetry/plugin-https of module https. Error: Cannot find module '@opentelemetry/plugin-https'
and
PluginLoader#load: could not load plugin @opentelemetry/plugin-express of module express. Error: Cannot find module '@opentelemetry/plugin-express'

By adding those 2 plugins to the `npm install` I got that step working.

Co-authored-by: Mayur Kale <mayurkale@google.com>

getting-started/README.md

@@ -54,13 +54,15 @@ This guide uses the example application provided in the `example` directory, but
 
 ([link to TypeScript version](ts-example/README.md#install-the-required-opentelemetry-libraries))
 
-To create traces on NodeJS, you will need `@opentelemetry/node`, `@opentelemetry/core`, and any plugins required by your application such as gRPC, or HTTP. If you are using the example application, you will need to install `@opentelemetry/plugin-http`.
+To create traces on NodeJS, you will need `@opentelemetry/node`, `@opentelemetry/core`, and any plugins required by your application such as gRPC, or HTTP. If you are using the example application, you will need to install `@opentelemetry/plugin-http`, `@opentelemetry/plugin-https` and `@opentelemetry/plugin-express`.
 
 ```sh
 $ npm install \
   @opentelemetry/core \
   @opentelemetry/node \
-  @opentelemetry/plugin-http
+  @opentelemetry/plugin-http \
+  @opentelemetry/plugin-https \
+  @opentelemetry/plugin-express
 ```
 
 #### Initialize a global tracer