Made typo correction on NPM installation command. (#6)

Author: mikebronner

README.md

@@ -37,7 +37,7 @@ composer require georgeboot/laravel-echo-api-gateway
 
 yarn add laravel-echo-api-gateway
 # or
-npn install --save laravel-echo-api-gateway
+npm install --save-dev laravel-echo-api-gateway
 ```
 
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