docs: highlight API key in Quick Start example (#1014)

Replaces the invalid API key in the first example with an env-variable, making it clear it has to be provided by the user.

fixes #1013 
---------

Co-authored-by: Martin Schuhfuss <m.schuhfuss@gmail.com>

README.md

@@ -41,7 +41,7 @@ This library is designed for server-side Node.js applications. Attempting to use
 
 Below is a simple example calling the elevation method on the client class.
 
-Import the Google Maps Client using Typescript and ES6 module:
+Import the Google Maps Client using TypeScript and ES6 module:
 
 ```js
 import {Client} from "@googlemaps/google-maps-services-js";
@@ -61,7 +61,7 @@ client
   .elevation({
     params: {
       locations: [{ lat: 45, lng: -110 }],
-      key: "asdf",
+      key: process.env.GOOGLE_MAPS_API_KEY,
     },
     timeout: 1000, // milliseconds
   })