fix typo, update readme with usage

Author: hapticdata

README.md

@@ -84,6 +84,14 @@ export function Altair() {
 }
 ```
 
+## Usage
+
+Add your Gemini API key to `.env` then:
+
+```
+$ npm install && npm start
+```
+
 ## development
 
 This project was bootstrapped with [Create React App](https://github.com/facebook/create-react-app).

src/App.tsx

@@ -24,7 +24,7 @@ import cn from "classnames";
 
 const API_KEY = process.env.REACT_APP_GEMINI_API_KEY as string;
 if (typeof API_KEY !== "string") {
-  throw new Error("set REACT_APP_GEMINI_APIK_KEY in .env");
+  throw new Error("set REACT_APP_GEMINI_API_KEY in .env");
 }
 
 const host = "generativelanguage.googleapis.com";