Update README headers

Author: corbindavenport

README.md

@@ -43,7 +43,7 @@ Package available from [packagist](https://packagist.org/packages/serpapi/serpap
 composer require serpapi/serpapi-php
 ```
 
-## Simple Usage
+## Simple usage
 
 ```php
 require 'vendor/autoload.php';

README.md.erb

@@ -67,7 +67,7 @@ Package available from [packagist](https://packagist.org/packages/serpapi/serpap
 composer require serpapi/serpapi-php
 ```
 
-## Simple Usage
+## Simple usage
 
 ```php
 require 'vendor/autoload.php';